Neoantigenic determinants (neoAg) specific for the assembling membrane attack complex (MAC) of complement were detected by immunofluorescence microscopy on the surface of cytotoxic lymphocytes during the antibody-dependent cellular cytotoxicity (ADCC) reaction. This study employed antibody-sensitized chicken erythrocytes as target cells, human peripheral blood lymphocytes as effector cells, and RITC-conjugated rabbit F(ab')2-anti-neoAg. NeoAg was present on 60% of ADCC plaque-forming lymphocytes (PFL). Eight out of 182 neoAg-positive PFL were observed in direct contact with their target cells. In these cases MAC-specific neoAg was visualized at the zone of contact between the cells. Anti-neoAg Ig was found to inhibit ADCC plaque assays up to 62%; and 51Cr-release assays up to 79%. Stimulation of lymphocytes by PHA or mixed lymphocyte culture increased the expression of neoAg. In the case of PHA, increased neoAg expression was correlated with an increased incorporation of 14C-leucine into C5, C6, C7, and C8 antigens, which was detected by immunodiffusion and autoradiography.