The O2- production as a marker of the respiratory burst was investigated under various stimulations in polymorphonuclear leukocytes of healthy young and aged subjects. Stimulation of the respiratory burst in the cells of elderly by specific agents (opsonized zymozan, N-formyl-methionyl-leucyl-phenylalanine, carbachol) resulted in a diminished response while it remained unchanged on the effect of non-specific stimulation (A23187, phorbol myristate acetate) comparing to young subjects. To elucidate the postreceptor signal transduction mechanism involved in respiratory burst stimulation various inhibitors were used as follows: neomycin (for phospholipase C enzyme), mepacrine (for phospholipase A2 enzyme) and pertussis toxin (for GTP binding regulatory protein). The results suggest that phospholipase C as well as phospholipase A2 could be involved in the postreceptor signal transduction depending on the stimulus, but the impairment of the pertussis toxin sensitive GTP binding protein with aging might explain the decrease response of the respiratory burst after stimulating the different receptors.