1. beta-Hexosaminidase (hex) structure was compared in various primates, using thin-layer isoelectric focusing on polyacrylamide gels and quantitative microcomplement fixation. 2. Isoelectric focusing revealed no intraspecies differences and similar interspecies patterns. 3. Hex A and B are evolving at a moderate, but equal, rate and in a manner consistent with accepted phylogenetic patterns. 4. Quantitative microcomplement fixation revealed a closer homology between human hex A and chimpanzee A or human hex B and chimpanzee hex B than between human hex A and human hex B.