All varieties of circulating white blood cells in patients with Chediak-Higashi syndrome characteristically contain giant granules in their cytoplasm. The similarity of the massive particles to normal-sized leukocyte lysosomes has been previously recognized. In the present study the technics of ultrastructural histochemistry were used to demonstrate acid phosphatase activity in giant C.H. particles and their degenerating remnants in the leukocytes of two patients with the C.H. syndrome. The presence of an acid hydrolase in massive granules indicates that they are lysosomes. The selective localization of enzyme reaction product in large particles by this method, when small normal-appearing lysosomes in the same cells remained unstained, has suggested that the unit membranes surrounding C.H. granules are abnormally permeable. The increased permeability of the large particles may be related to the pathogenesis of morbid clinical features of C.H. syndrome.