Lysozyme from human saliva was purified in one step by affinity chromatography on chitin. The recovery, however, was always very low, typically 25-40%. When the cationic exchanger BioRex 70 was used, the enzyme was isolated from saliva with only minor loss of activity, and the product appeared to be 60-70% pure. This partially purified enzyme bound completely and reversibly to chitin and was thereby purified to homogeneity with little loss of activity (< 10%). Both saliva lysozyme and the chicken egg white lysozyme were found to exhibit their highest lytic activity at high pH/low ionic strength (pH 9.0, I = 0.03), and both enzymes required higher ionic strength) it was found, however, that the ratio of the specific activity of saliva lysozyme to that of chicken lysozyme varied considerably depending on the assay conditions used. We argue strongly, therefore, against the frequent use of chicken lysozyme as a standard of reference in work on human lysozyme.