The ability of neutrophils to phagocytose, kill and digest Candida guilliermondii was investigated in twelve patients with chronic granulocytic leukaemia (CGL). Following ingestion of organisms there was considerable reduction in the ability of CGL cells to kill Candida and this was not explained by a mild impairment of phagocytosis. Histochemical staining showed that granules containing lysosomal enzymes disappear form the cytoplasm of normal neutrophils during killing and digestion of the fungus, while in CGL cells the granules remain. Quantitative measurement confirmed loss of peroxidase from normal neutrophils during Candida killing but no loss from CGL neutrophils. The primary granules of CGL neutrophils do not fuse with and discharge their contents into the phagocytic vacuole and this explains their impaired ability to kill and digest Candida.