Phagocytic capacity of circulating neutrophils and monocytes was studied by electron microscopy in 2 children with Chédiak-Higashi syndrome (CHS). Apparent phagocytosis of autologous leucocytes by the CHS neutrophils and monocytes was demonstrated when they were coincubated with Staphylococcus aureus in vitro, indicating their capacity for hyperactive phagocytosis. The hyperactive phagocytic capacity of the CHS phagocytes was a cellular abnormality. CHS neutrophils and monocytes are known to be defective in chemotaxis and bactericidal capacity, and the hyperactive phagocytic capacity is another functional abnormality of CHS phagocytes.