The generation and hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) have been shown to occur early after the stimulation of a variety of cells and to trigger an array of responses, including proliferation in lymphocytes. In this study we examined the requirement for macrophages in the production and metabolism of PIP2 in lectin-stimulated lymph node lymphocytes. When lymph node cells (LNC) were stimulated by the mitogenic lectin, Con A, a two- to threefold increase in [3H]inositol-labeled PIP2 occurred within 15 min, followed by a decrease to the level of unstimulated cells by 30 min. In addition, there was also a two- to threefold increase in the accumulation of [3H]inositol-labeled inositol monophosphate (IP) 30 min after the addition of lectin. However, when the LNC were depleted of macrophages by adherence to plastic and passage over cotton columns before the addition of Con A, the PIP2 response was not seen and the IP accumulation was reduced by 40%. The macrophage-depleted, Con A-stimulated cultures failed to synthesize DNA. [3H]TdR incorporation was only 15% of that of Con A-stimulated unseparated LNC cultures. Soluble IL-2 production was also depressed to 20% of that observed in macrophage-containing cultures. Furthermore, when macrophages (2%) were added to the macrophage-depleted cultures, PIP2 production, IP accumulation, and [3H]TdR incorporation were restored to levels observed in the stimulated unseparated LNC cultures. The addition of IL-1 or the phorbol ester 12-O-tetradecanoylphorbol-13-acetate in combination with Con A also restored both [3H]TdR incorporation and IL-2 synthesis in macrophage-depleted cultures. However, they did so without a detectable increase in phosphatidylinositol metabolism. Collectively, these data suggested that the production and degradation of PIP2 in Con A-stimulated lymphocytes was potentiated by the presence of macrophages.