In vitro Ca++ activates gelsolin to sever F-actin and form a gelsolin-actin (GA) complex at the + end of F-actin that is not dissociated by ethylene glycol-bis(3-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) but is separated by EGTA + PIP/PIP2. The gelsolin blocks the + end on the actin filament, but the -end of the filament can still initiate actin polymerization. In thrombin activated platelets, evidence suggests that severing of F-actin by gelsolin increases GA complex, creates one -end actin nucleus and one cryptic + end actin nucleus per cut, and then dissociates to yield free + ends to nucleate rapid actin assembly. We examined the role of F-actin severing in creation and regulation of nuclei and polymerization in polymorphonuclear neutrophils (PMNs). At 2-s intervals after formyl peptide (FMLP) activation of endotoxin free (ETF) PMNs, change in GA complex was correlated with change in + end actin nuclei, -end actin nuclei, and F-actin content. GA complex was quantitated by electrophoretograms of proteins absorbed by antigelsolin from cells lysed in 10 mM EGTA, + end actin nuclei as cytochalasin (CD) sensitive and -end actin nuclei as CD insensitive increases in G-pyrenyl actin polymerization rates induced by the same PMNs, and F-actin content by NBDphallacidin binding to fixed cells. Thirty three percent of gelsolin was in GA complex in basal ETF PMNs; from 2-6 s, GA complexes dissociate (low = 15% at 10 s) and sequentially + end nuclei and F-actin content and then -end nuclei increase to a maximum at 10 s. At >40 s GA complex increase toward basal and + end nuclei and F-actin content returned toward basal. These kinetic data show gelsolin regulates availability of + end nuclei and actin polymerization in FMLP. However, absence of an initial increase in GA complex or -end nucleating activity shows FMLP activation does not cause gelsolin to sever F-or to bind G-actin to create cryptic + end nuclei in PMNs; the results suggest the + nucleus formation is gelsolin independent.