The objective of this study was to elucidate the mechanism responsible for inhibition as well as induction of cytolytic activity in cytotoxic T lymphocytes (CTL) by cluster-defined 3 (T3) (CD3) monoclonal antibodies (mAb). A series of isotype heavy chain switch variants (murine IgG1, IgG2a, IgG2b, IgA, and IgE) of a single CD3 mAb was used in the analysis of effects of CD3 mAb on the cytolytic activity of an allospecific CTL clone. The results obtained indicate that the inhibition of cytolytic activity of CTL by CD3 mAb is not caused by interference with events that occur after conjugate formation, such as programming for lysis and the delivery of the lethal hit, but by inhibition of specific recognition of the target cell by the CTL. We also present evidence that induction of nonspecific CTL activity by CD3 mAb is only achieved by direct binding of the Fc receptor to the CD3 mAb. This observation allowed us to use the CD3 mAb-induced nonspecific cytolytic activity of CTL as a sensitive assay for detection of Fc receptors for various heavy chain isotypes. So far, evidence was found for human Fc receptors reactive with murine IgG1, IgG2a, and IgG2b, but not for murine IgA or IgE. The combined results indicate a similar role for Fc receptors in induction of cytolytic activity of CTL by CD3 mAb as well as in CD3 mAb-induced proliferative responses of T cells.