A method has been developed which covalently attaches biotin to proteins on the outer surface of leukocytes. These proteins are separated by polyacrylamide gel electrophoresis and transferred to nitrocellulose by protein blotting. Labeled proteins are detected using an avidin-peroxidase conjugate and color indicator. The method has been used to identify surface proteins specific to either polymorphonuclear leukocytes or mononuclear leukocytes prepared from bovine peripheral blood. The method provides a highly sensitive, non-radioactive means of examining alterations of surface proteins on leukocytes under differing functional and physiological conditions. Cell viability is not altered by this labeling method and labeled cells can be used to examine functions of surface proteins.