As the published values for the molecular weight of L-alpha-hydroxyacid oxidase vary from 89 000 to 430 000, it is possible that such variations could be due to a concentration dependence of the molecular weight. The molecular weight of rat L-alpha-hydroxyacid oxidase was studied over a wide range of concentrations, using equilibrium sedimentation and gel exclusion chromatography. The partial specific volumes (0.726 and 0.730 for hydroxyacid oxidase A and hydroxyacid oxidase B, respectively) were calculated from the amino acid compositions, and were used to calculat molecular weights from the equilibrium sedimentation data. The molecular weight at infinite dilution was found to be 150 000 for both the A and B isozymes. Both isozymes exhibit association-dissociation behaviour at low concentrations. The self-association of the hydroxyacid oxidase B isozyme can be described by the relation (see article) where K1,2 = 5.4-10(5) M-1 and K2,4 = 1.7-10(5) M-1. Previously published values of the molecular weight of these isozymes are in accord with the observed concentration dependence.