Treatment of guinea pig neutrophils with pertussis toxin (islet-activating protein; IAP) results in inhibition of N-formyl peptide receptor-mediated release of arachidonic acid and granular enzymes. Inhibition by the toxin is specific, in that responses to the calcium ionophore A23187 are not affected. The action of the toxin is not associated with alterations in cellular concentrations of cyclic AMP but is correlated with the ability of the toxin to catalyze the ADP-ribosylation of a 41,000 dalton membrane protein. This protein comigrates on SDS-polyacrylamide gels with the (Y subunit of Gi, the inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase. It is likely that this G protein is involved in receptor-mediated signal transduction in neutrophils by mechanisms that do not involve cyclic AMP.