The action of the 88K protein/actin complex (88K/A) and cytochalasin B on various aspects of actin polymerization kinetics was investigated, and the results were interpreted in terms of the condensation polymerization and treadmilling mechanism for actin polymerization. A substoichiometric concentration of 88K/A promotes actin nucleation under physiological salt conditions, especially in the presence of Ca2+. In addition, it reduces both the elongation rate and the depolymerization rate by up to 70% and inhibits annealing of the actin filaments. As a consequence, the average length of actin filaments polymerized with 88K/A becomes less than that of a control. These data indicate that 88K/A caps one end of actin filaments or actin oligomers where in the absence of 88K/A the rates for both association and dissociation of monomers are faster than at the other end. In a KCl/MgCl2 medium, 88K/A increases the steady state monomer concentration (the critical concentration) to a limited extent. This is explained by assuming that 88K/A caps the lengthening end (in treadmilling) of actin filaments, where the critical concentration is lower than at the other end (the shortening end). Moreover, cytochalasin B which has been shown to bind to the barbed end of actin filaments does not affect the 88K/A-nucleated actin polymerization. Therefore, it is strongly suggested that 88K/A caps the barbed end of actin filaments and that the barbed end is the lengthening end as well as the rapidly growing and rapidly depolymerizing end. The result obtained in the study on the action of cytochalasin B was consistent with this suggestion.