Conflicting data suggest that profilin might function to promote either actin polymerization or depolymerization in cells. There are theoretical reasons and supportive data to suggest that profilin might do both. Perhaps the most accurate description of profilin emphasizes its ability to augment actin-filament dynamics, both in polymerization and in depolymerization. The effect of profilin on the critical concentration of actin, its ability to depolymerize filaments at the barbed end and the formation of a ternary complex with thymosin beta(4) all need to be accurately represented in any attempt to determine a model for profilin function.