Activation of T lymphocytes through the T-cell receptor and some co-stimulatory molecules proceeds in a stepwise fashion resulting in activation of phospholipase C-y (PLC-y) isozymes with subsequent hydrolysis of membrane inositol phospholipids. We evaluated the early stages of lymphocyte activation by measuring phosphatidylinositol (PI) hydrolysis resulting from surface receptor crosslinking. We permeabilized freshly isolated purified human T lymphocytes with staphylococcal a-toxin to facilitate incorporation of [Hl-myoinositol into membrane phospholipids. Aggregation of surface receptors (TCR-CD3 complex and CDS) with specific antibodies produced hydrolysis of inositol phospholipids as measured by release of soluble inositol phosphates into the aqueous phase. Orthovanadate, the positive control, consistently induced hydrolysis of >50% of the inositol phospholipid pool in permeabilized cells. Using cells from normal volunteers, low dose plastic adherent anti-CD3 antibodies (250 ng/ml) produced 4.8 + 2.0% net PI hydrolysis, while plastic adherent anti-CD5 antibodies (250 ng/ml) produced 3.8 + 0.9%. Simultaneous CD5/CD3 crosslinking produced 16.3 + 1.0% net PI hydrolysis representing a synergistic increase (p<0.035). This indicates that CDS-mediated PLC activation is independent of that produced by CD3 and provides an important T cell activation co-stimulus.