The factors involved in the microbial degradation of polyesters have been studied using a series of polycaprolactones as model compounds. A variety of microorganisms has been isolated by enrichment culture, capable of degrading members of the series. It appears that the enzyme systems involved in the degradation vary among the organisms tested. Analysis of enzyme degradation products indicates that although initial degradation is accomplished by extracellular enzymes, some organisms utilize cell associated enzymes to complete the degradation. In addition, a number of microorganisms produce a low molecular weight substance capable of enhancing the activity of the esterases involved in depolymerization. The material is heat labile and passes through an ultrafilter with a molecular weight cutoff of 500. It has no inherent depolymerase activity but nonspecifically enhances enzymes from a number of other microorganisms. A linear dose response is seen when increasing amounts of cofactor are added to a constant amount of enzyme.