The characteristics of homotypic neutrophil aggregation, mediated by the adhesion molecule CD11b/CD18, differ according to whether activation takes place via intracellular protein kinase C(PKC) inducers or chemoattractants. In response to diacylglycerol (DAG) analogues such as PMA and 1,2-dioctanoyl-sn-glycerol, a prolonged cellular aggregation occurs that is associated with intense phosphorylation of the CD18 beta-chain. In response to the chemoattractant FMLP, a more transient aggregation event results that is associated with minimal beta-chain phosphorylation. By using the PKC inhibitor staurosporine, we now show that these differences are likely to reflect two different pathways of activation. Both aggregation and phosphorylation induced by DAG analogues are completely abolished by staurosporine in a parallel dose-dependent manner. Conversely, FMLP-induced aggregation is enhanced and prolonged by staurosporine whereas the associated minimal phosphorylation event is further diminished by staurosporine. Accordingly, activation of neutrophil aggregation by DAG analogues is associated with and presumably due to phosphorylation of the CD18 beta-chain. This intense phosphorylation occurs via a staurosporine-sensitive kinase such as PKC. FMLP, on the other hand, appears to activate CD11b/CD18 by a distinct mechanism. This latter mechanism does not seem to be dependent on what may be a minor PKC-induced phosphorylation of the beta-chain, and indeed is enhanced by inhibition of PKC. Of note, staurosporine was also found to cause selective release of specific granules with concomitant increase in surface display of CD11b/CD18. These data further support previous observations that up-regulation of this adhesive molecule is not the primary event in the induction of cellular adhesiveness.