The density-distribution patterns of various enzymes and of labeled materials have been determined by isopycnic centrifugation in a sucrose—0.2 M KCl gradient on homogenates of lymphoid tissues from rats injected with Triton WR-1339, 14C-labeled dextran, 51Cr-labeled erythrocytes, and cortisol. The results confirm and extend the conclusion, derived from previous investigations on normal animals, that the lysosomes of lymphoid tissues form two and possibly three, distinct populations. The evidence indicates that the L19 population belongs to macrophages and the L15 group to lymphocytes. The L30 population appears to be associated with a special type of phagocyte with a high capacity for dextran storage. All three populations seem to contribute to the activities found in soluble form in homogenates of normal lymphoid tissues.

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