We carried out a detailed analysis of the requirement for Ca2+ in the lysis of target cells by cloned cytotoxic T lymphocytes (CTL). In direct, antigen-specific lysis we always observed an influx of Ca2+ into the CTL concomitant with target cell binding. However, we never observed an increase in CTL Ca2+ content during lectin-mediated lysis, or nonspecific lysis by phorbol myristate acetate-induced CTL. We found that in all three types of lysis (direct, lectin-mediated lysis, C or phorbol myristate acetate-induced) the requirement for Ca2+ in lysis was dictated by the target cell used; the same CTL can kill one target cell in the absence of detectable Ca2+, and absolutely require Ca2+ for the lysis of another target cell. Target cell killing, when it occurred in the absence of Ca2+, was accompanied by microtubule organizing center reorientation in the CTL, showing that this function is not uniformly Ca2+ dependent. These results provide further evidence that Ca2+ is not always required for activation of the lytic pathway in CTL, although Ca2+ may be absolutely required for other CTL functions such as interleukin production or expression of the interleukin 2 receptor.