26 specimens of Presbytis entellus were examined for a variety of blood proteins. In contrast to previous studies of other species of leaf monkeys, our P. entellus sample proved to be very heterogeneous. Polymorphisms were found in the third component of complement, group-specific component, glycine-rich β-glycoprotein, α1-antitrypsin, phosphoglucomutase, 6-phosphogluconate dehydrogenase, adenylate kinase, superoxide dismutase, malate dehydrogenase, and phosphohexose isomerase. Variable band strengths that might represent polymorphism were found in acid phosphatase and lactate dehydrogenase. Further analyses of the P. cristatus sample studied by Barnicot and Hewett-Emmett failed to disclose variation. The interpretation of blood protein variability in relation to sample collection and population structure is discusse