Block of activating receptors reveals NKG2A-mediated protection from NK lysis. The indicated tumor cell lines were tested as targets in a 51Cr release assay using CD56+/CD3- cells as effectors, in the presence of the indicated antibodies. Blocking antibodies to activating receptors were selected on the basis of the ligands expressed by the target: A431 (DNAM-1), End7 (NKG2D + DNAM-1), SK-MEL 37 (DNAM-1), and Ova7 (NKG2D+DNAM-1). Differences in 51Cr release values were statistically significant when the average values of triplicates differed from control more than three times the SD, that is, at E/T ratios from 20:1 to 2.5:1 in A431: donor 1 as well as End7:donor 1 and at E/T ratios of 20:1 and 10:1 in A431:donor 3. The top two panels display two E/T combinations in which the inhibitory effect of NKG2A becomes evident only when activating receptors are antibody-blocked. End7:donor 1 exemplifies a case in which NKG2A similarly protects in the presence and absence of antibody block. In the remaining cases, activating receptors and NKG2A cannot be demonstrated to influence susceptibility to lysis.