To investigate a possible function of the nervous tissue-specific protein kinase C substrate B-50/GAP-43 in regulation of the dynamics of the submembranous cytoskeleton, we studied the interaction between purified B-50 and actin. Both the phosphorylated and dephosphorylated forms of B-50 cosedimented with filamentous actin (F-actin) in a Ca(2+)-independent manner. Neither B-50 nor phospho-B-50 had any effect on the kinetics of actin polymerization and on the critical concentration at steady state, as measured using pyrenylated actin. Light scattering of F-actin samples was not increased in the presence of B-50, suggesting that B-50 does not bundle actin filaments. The number of actin filaments, determined by [3H]cytochalasin B binding, was not affected by either phospho- or dephospho-B-50, indicating that B-50 has neither a severing nor a capping effect. These observations were confirmed by electron microscopic evaluation of negatively stained F-actin samples, which did not reveal any structural changes in the actin meshwork on addition of B-50. We conclude that B-50 is an actin-binding protein that does not directly affect actin dynamics.