Engagement of the antigen-specific receptor (TCR) of CD4+ T lymphocytes without a second (costimulatory) signal prevents the subsequent production of interleukin-2 (IL-2) by these cells. Because IL-2 is a key immunoregulatory lymphokine and is also produced by a subset of CD8+ T cells that are able to kill target cells, the effect of engaging the TCR of one such clone in the absence of costimulatory signals was examined. The capacity for TCR-dependent IL-2 production was lost, indicating comparable costimulator-dependent signaling requirements for IL-2 production in CD4+ and CD8+ T cells. However, TCR-mediated cytotoxicity was not impaired, implying that costimulation is required for only certain TCR-dependent effector functions.