Granulocyte function was evaluated using peritoneal exudate granulocytes from a strain of beige mice with the Chediak-Higashi syndrome (CHS). Defective granulocyte chemotaxis of CHS cells (43% normal) was documented. There was no abnormality in the ability of CHS serum to function as a chemotactic stimulus. Phagocytosis of 14C-labeled Staphylococcus aureus by CHS granulocytes was normal. However, there was reduced bactericidal activity of S. aureus and group D streptococcus by CHS cells through 90 min of incubation. The bactericidal defect was most pronounced at early time periods and was related to impaired intracellular killing. These defects of CHS mice granulocytes were analogous to those reported in man and it is concluded that the CHS mouse is a most convenient and representative model of the CHS of man.