We undertook this study to determine the quality, quantity, and temporal relationship of pep M5-induced cytokine release. The ability of pep M5 to stimulate interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-a) production by a T-cell-depleted, monocyte-and B-cell-enriched cell population was dependent on the presence of T cells. The requirement for T cells could be met by addition of exogenous gamma interferon (IFN-y). In the presence of IFN-y, pep M5 induced the release of TNF-a, IL-1, and IL-6. TNF-ae levels peaked at 24 h, while IL-1 and IL-6 levels peaked at 48 h. pep M5 induced T cells to produce IFN-y, which may have accounted for the ability of the superantigen to induce the production of IL-1, IL-6, TNF-c, and TNF-0 by peripheral blood mononuclear cells (PBMC). The addition of excess IFN-y to cultures of pep M5 and PBMC did not further increase the release of these cytokines at 24 and 48 h but resulted in sustained higher levels at 72 h. Interestingly, TNF-1 production occurred only in the presence of pep M5 and exogenous IFN-y. The * Corresponding author.