El contexto es fundamental para llegar al significado exacto, el contexto que proporcionaste es muy escueto. Pero en el link incluído encontraras más contexto. Tenés que buscar por "cold target inhibition" ya que es una técnica de ensayo de investigación de agentes patógenos. NK stands for natural killer as you will see in the technical article.
Target cell lysis was assessed by a 51Cr release assay as described previously (54). Target cells were labeled with Na251CrO4 (New Life Science Products, Inc., Boston, Mass.) for 1 h. The target cells (104) were placed in wells of 96-well round-bottomed microtiter plates. Effector cells were used at a concentration of 106/100 μl, and threefold serial dilutions were made to provide effector-to-target cell (E/T) ratios of 100:1, 33:1, 11:1, and 3:1. After a 5-h incubation, the radioactivity released from the cells was measured in a model 20/20 gamma counter (Iso-Data, Inc., Palatine, Ill.). The percentage of target cell lysis was calculated by using the following equation: % lysis = [(experimental release cpm − spontaneous release cpm)/(maximal release cpm − spontaneous release cpm)] × 100.
In cold target inhibition experiments, 106 effector cells were incubated for 5 h with 104 51Cr-labeled PSJL cells in the presence of various numbers of nonlabeled cold inhibitor PSJL, BxSF1, C57SV, or EL4 cells (37).
To analyze a pathway for cell lysis, effector cells were preincubated with either brefeldin A (PharMingen, San Diego, Calif.) or concanamycin A (Sigma, St. Louis, Mo.) as an inhibitor for a Fas-FasL or a perforin pathway, respectively (11, 21, 26). A negative control group was incubated with dimethyl sulfoxide, since both brefeldin A and concanamycin A were dissolved in dimethyl sulfoxide as stock solutions.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC136689/

http://jem.rupress.org/content/188/12/2375.full.pdf

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We have previously shown that human cultured trophoblast cells are resistant to lysis by natural killer (NK) cells from both peripheral blood and decidua although cells are present in decidua which do exhibit NK activity against K562 (1). Using a cold-target inhibition assay and a single-cell conjugate assay we have now examined whether these trophoblast cells have NK target structures on their surfaces. Our findings indicate that first-trimester human trophoblast cells do not express surface structures recognized by decidual Leu 19+ (CD56+) large granular lymphocytes (LGLs) isolated from human decidua. Immunostaining of the conjugates formed between decidual NK effectors and K562 cells confirmed that these effector cells are CD56+ LGLs.
http://www.sciencedirect.com/science/article/pii/00088749909...

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Células no marcadas radioactivamente. (aporte de lugoben)
Células no radiomarcadas. (aporte de alberto jiménez) ambas son buenas traducciones.

La penetración en los capilares de las células neoplásicas puede verse ... Estudios de distribución de células tumorales marcadas radiactivamente han.....
https://books.google.es/books?id=-FRNFXtgAigC&pg=PA31&lpg=PA...

Porcentaje de mitosis marcadas (PMM): se marcan células por un período breve, ... con un precursor del ADN marcado radiactivamente (en general timidina)
https://books.google.es/books?id=ds5zPSJ-3RkC&pg=PA248&lpg=P...

mentó a las células que han reaccionado con el anticuerpo debería matarlas, ... la célula T Si se incuban células con aminoácidos marcados radiactivamente..
https://www.google.es/?gws_rd=ssl#q=células no marcadas radi...

Si las células nadadoras están marcadas radiactivamente de manera uniforme por crecimiento con glucosa tritiada, y son después transferidas a un medio con....
https://www.google.es/?gws_rd=ssl#q=células no marcadas radi...



The results of this paper provide the theoretical basis for "cold" (unlabeled) target inhibition of radioactive assays of lymphocyte-mediated cytolysis
http://www.ncbi.nlm.nih.gov/pubmed/111379

In cold target inhibition experiments, nonradioactive (cold) target cells and 5'Cr-labeled (hot) target cells at different cold/hot ratios were incubated with effector cells at fixed E/T ratio

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Note added at 11 hrs (2015-07-13 12:42:31 GMT)
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missing reference for "in cold target inhibition experiments
http://www.bloodjournal.org/content/bloodjournal/83/4/1060.f...

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Note added at 13 hrs (2015-07-13 14:27:59 GMT)
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Corrección
la palabra en el término propuesto "radioactvemente" cambiar por "radioactivamente"