LYSOZYME has been detected in the milk of donkey, mare, bitch, sow and cow1,2. In order to investigate the possible biological significance of lysozyme an investigation is planned here to compare physical, chemical and physiological characteristics of the lysozymes isolated from bovine and human milk. Although several workers1-4 have reported the presence of lysozyme in human milk and colostrum, not much work has been done to obtain information on the quantitative amounts of the enzyme in this milk. In the work recorded here lysozyme contents of a large number of bovine and human milk samples were determined. In an earlier publication2 the lysozyme content of bovine milk was reported to vary between 0.0 and 260.0 µg with an average of 13 µg of the enzyme activity per 100 ml. of milk, using egg-white lysozyme (Difco) as the standard. This report deals with the lysozyme content of human milk and colostrum, and the effect of storage and heat on the enzyme.