Granules from rabbit peritoneal leucocytes were prepared in 0.3 M sucrose as an optically homogeneous suspension with the aid of heparin. Lysis of the granules in vitro was followed by measurement of decreases in the apparent absorbance of the suspensions at 520 mµ and was accompanied by solubilization of beta-glucuronidase from the particles. Streptolysins O and S from hemolytic streptococci lysed the granules at 20°C; the initial rate of lysis by streptolysin O was greater than that by streptolysin S. Cysteine activated, and specific antibody inhibited, streptolysin O; antimycin and bovine serum albumin inhibited streptolysin S. The granules were not lysed by any other streptococcal exotoxins. Lysis was irreversible and depended neither upon oxidative phosphorylation, nor upon intact respiration. The granules were also lysed by lysolecithin, at concentrations from 2 x 10-6 M to 1 x 10-4 M; bovine serum albumin and antimycin also inhibited this lytic agent. Such other hemolytic agents and procedures as vitamin A, non-ionic detergents, and ultraviolet irradiation also disrupted leucocyte granules. In susceptibility to lysis and other properties, the granules of white cells resembled erythrocytes. Leucocyte granules differed from mitochondria in that they did not appear to take up or extrude water reversibly; they were unaffected by thyroxine, phosphate, or metabolic substrate. The studies are compatible with the hypotheses that white cell granules are similar to lysosomes isolated from other tissues, and that they share common surface properties with erythrocytes.

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