Porins are trimeric proteins that constitute water-filled pores that allow transmembrane diffusion of small solutes through the outer membrane layer of gram-negative bacteria. The porins are capable of inserting into the membranes of eucaryotic cells, and in the present study we have examined the in vitro effects on neutrophil functions of the following purified porins: meningococcal outer membrane protein classes 1 and 3 and gonococcal outer membrane protein 1B (P1B). The neisserial porins inhibited human neutrophil chemoattractant-induced actin polymerization and degranulation of both primary and secondary granules. The neutrophil expression of immunoglobulin G (IgG) Fc receptors II (Fc gamma RII; CDw32) and III (Fc gamma RIII; CD16), as well as the activation-dependent downregulation of Fc gamma RIII, were reduced by the meningococcal and gonococcal porins. The neisserial porins impaired the upregulation of complement receptors 1 (CD35) and 3 (CD11b) and inhibited the phagocytic capacity of neutrophils, as evaluated by the uptake of meningococci (strain 44/76) in the presence of patient serum containing known amounts of IgG against meningococcal porins. The porins also primed neutrophils to increase their intracellular hydrogen peroxide production in response to FMLP, whereas no such priming was observed if the neutrophil protein kinase C was stimulated directly with phorbol myristate acetate. The neisserial porins influenced neutrophil functions in a time- and concentration-dependent manner. The meningococcal class 1 outer membrane protein and the gonococcal P1B tended to alter neutrophil functions more than the meningococcal class 3 protein. Thus, the neisserial porins inhibited human neutrophil actin polymerization, degranulation, opsonin receptor expression, and phagocytosis but primed the neutrophils to increase their oxidative burst. It remains to be determined whether these in vitro observations reflect mechanisms that may be of importance for the interaction between neutrophils and Neisseria species in vivo.