In the preceding paper [Golsteyn & Waisman (1989) Biochem. J. 257, 809-815] an EGTA-stable, Ca2+-binding heterodimer comprised of a 50 kDa protein and actin called '50K-A' was identified in the unfertilized eggs of the sea urchin Strongylocentrotus purpuratus. In the present paper we have documented the binding of 50K-A to DNAase I and the effect of 50K-A on the kinetics of actin polymerization. When 50K-A was added to pyrene-labelled rabbit skeletal-muscle actin and the salt concentration increased, the initial rate of actin polymerization was inhibited by a very low molar ratio of 50K-A to actin. Furthermore, the steady-state level of G-actin was increased in the presence of 50K-A, suggesting that 50K-A caps the preferred end of actin polymer, shifting the steady-state concentration to that of the non-preferred end. Dilution of F-actin to below its critical concentration into 50K-A resulted in a much slower rate of depolymerization, consistent with capping of the preferred end. In contrast with the Ca2+-dependent binding to DNAase, the effect of 50K-A on the kinetics of actin assembly and disassembly was Ca2+-independent. These results suggest that 50K-A is a novel actin-binding protein with some similarities to the severin/fragmin/gelsolin family of F-actin-capping proteins.