When stimulated by phorbol 12-myristate 13-acetate (PMA), superoxide generation in neutrophils from old volunteers was modestly lower than neutrophils from young subjects. PMA receptor number and affinity were normal. Protein kinase C (PKC) translocation to the membrane was normal but its activation was reduced. PMA-induced total endogenous phosphorylation and phosphorylation of individual proteins showed no age-related differences as determined by SDS-PAGE analysis. These minimal alterations in neutrophil function contrast with the much more significant decrements in superoxide generation and calcium homeostasis noted when neutrophils from old volunteers are stimulated by chemotactic peptide formyl-methionyl-leucine-phenylalanine (FMLP) (Lipschitz et al., 1988). It is well recognized that phorbol activates the cell through a mechanism that bypasses the membrane-receptor. Taken together with our observations with FMLP, these results point to a membrane-associated deficiency in the signal transduction pathway, most likely through receptor coupling or alterations in membrane lipids. They also demonstrate that there is not an overall reduction of metabolic responses in neutrophils from the elderly.