It has long been assumed that a rise in cytosolic free Ca2+, [Ca2+]i, is a necessary and sufficient event for the stimulation of a variety of cellular processes. The development of a technique which allows monitoring of [Ca2+]i in small intact cells has led to a critical revision of this simple postulate. We have recently shown that in neutrophils, Ca2+-ionophore-induced elevations of [Ca2+]i, quantitatively similar to those caused by chemotatic peptides, are ineffective in stimulating cell responses, which suggests that an additional signal is required for receptor-mediated activation. Here we show that subthreshold concentrations of phorbol myristate acetate (PMA) and of a Ca2+ ionophore can quantitatively mimic the effect of a physiological agonist. However, PMA at higher concentrations can trigger NADPH-oxidase activity, exocytosis and protein phosphorylation, even when [Ca2+]i is lowered 10-20 times below the normal resting level. These results strongly suggest that activation of protein kinase C is sufficient, by itself, to induce NADPH-oxidase activation and exocytosis of secondary granules in neutrophils.