Cultured fibroblasts from a patient with the Chediak-Higashi syndrome, the mother of the patient, and a normal control were studied by light and electron microscopy. The distribution pattern of PAS-positive and acid phosphatase-containing granules in the cytoplasm differed significantly in the fibroblasts from the patient when compared with those from the mother and control. The granules in the fibroblasts from the patient were clustered in the perinuclear area, whereas the granules in the fibroblasts from the mother and control were dispersed throughout the cytoplasm. After incubation with ascorbic acid, the clustered granules in the fibroblasts of the Chediak-Higashi syndrome showed a tendency to spread throughout the cytoplasm. The distribution pattern of the granules was studied by quantitative morphology.