Stimulation of rat neutrophils with the peptide fMetLeuPhe caused (i) the appearance of a 40 kDa protein in the Triton-X-100-insoluble cytoskeleton, (ii) the disappearance of DNAase inhibition from the cytosol and (iii) the appearance of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phallacidin (NBD-phallacidin) binding sites. All three observations were consistent with a rapid and transient assembly of polymerized actin, peaking at approximately 5 s and returning to near resting levels within 40 s. By experimentally depleting the cells of Ca2+ and increasing the cytoplasmic Ca2+ buffering capacity, the peptide-induced Ca2+ transient was reduced from a peak of 900 nM to 250 nM, without inhibiting actin polymerization, and this peak was sustained for at least 2 min. A further dissociation between the triggering of actin polymerization and peptide-induced Ca2+ elevation and oxidase activation was demonstrated at low concentrations of peptide (1-100 pM), actin polymerization being triggered without an elevation in Ca2+ or activation of the oxidase. Two other agents which induced actin polymerization, phorbol 12-myristate 13-acetate and latex beads, failed to elevate cytoplasmic Ca2+. It was therefore concluded that neither Ca2+ nor those intracellular messengers which act with Ca2+ to trigger the neutrophil oxidase are responsible for triggering actin polymerization in neutrophils.