Perforin-dependent mechanism of granzyme B–independent cytotoxicity. Granzyme B^+/−, granzyme B^−/−, or perforin (^−/−) splenocytes were mixed with irradiated Balb/c splenocytes to generate H-2^b anti–H-2^d CTLs in day 5 MLR cultures. CTLs were tested against TA3 allotargets at various E:T ratios in 4-hour (A and B) and 10-hour (C and D) ⁵¹Cr release (A and C) and ¹²⁵IUdR release (B and D) assays. Granzyme B^−/− and perforin^−/− CTLs are equally unable to induce DNA fragmentation at 4 hours of incubation. Prolonged incubation of effectors and targets shows that the cytotoxicity remaining in granzyme B^−/−CTLs is due to both perforin-dependent and perforin-independent mechanisms. Error bars show the range of values at each point. This experiment represents one of four with essentially identical data.