1. The beta-N-acetylglucosaminidases studied from peripheral blood sera have the following activities: 55.6 mU/ml in Bos taurus L. (bull, "Morucha" race), 31.3 mU/ml in Sus scropha var. domestica L. (pig) and 3.7 mU/ml in Capra hircus L. (goat). 2. The pH optima of the enzymes are 4.5 for Bos taurus L. and Sus scropha L., and 5.0 for Capra hircus L. 3. Two enzyme forms, A and B, have been separated from Bos taurus L. and Capra hircus L. sera by DEAE-cellulose chromatography; and three forms, A, B and S, from Sus scropha L. serum. 4. The B forms are more heat-stable than the A and the S forms. The S form has intermediate heat-stability between the A and the B forms.