We have previously shown that interleukin-1 (IL-1) rapidly stimulates the hydrolysis of phosphatidylcholine in the human T lymphocyte cell line, Jurkat (Rosoff, et al., Cell 54: 73-81, 1988). This was apparently mediated by a phospholipase-C catalyzed mechanism, occurring initially at the outer plasma membrane. In this report, I have further characterized this activity of IL-1. The hydrolysis of phosphatidylcholine was dependent upon extracellular Ca2+, although it appeared to be relatively independent of Mg2+. The activity was totally inhibited by prior treatment of intact Jurkat cells with trypsin. In addition, treatment of Jurkat cells with a phosphatidylinositol-specific phospholipase C, which selectively removes proteins anchored by glycosyl-phosphatidylinositol linkages, completely blocked the ability of IL-1 to stimulate the hydrolysis of phosphatidylcholine. These data suggest that the initial activity of IL-1 is to stimulate a Ca2+-dependent, glycosyl-PI-anchored phospholipase C, the active site of which is on the extracellular surface.