Studies of L1210 Leukemia. II. Ultrastruc- tural and Cytochemical Changes After Treat-

SUMMARY-Transplantation of L1210 leukemic cells into DBA/2J mice produced a systemic leukemia which regressed after treatment with cyclophosphamide. Involution of the spleen was reflected histochemically by disappearance of leukemic cells and an aHlux of lysosome-rich macro-phages, and, biochemically, by an increase in acid phosphatase activity. The increase in acid phosphatase activity in spleens from mice treated concomitantly with cyclophosphamide and vitamin A appeared enhanced. Numerous multinucleated giant cells were observed during the period of leukemia regression. Some were megakaryocytes, possibly representing reservoirs for the virus particles normally present in the leukemic cells. The role of lysosomal enzymes derived from the invading macrophages in the degradation of tumor cells, and the potentiality of lysosomal labilixers to enhance the degradative process of leukemic cells are discussed.-J Nat Cancer Inst 39: 385-42L 1967. THE ROLE OF lytic enzymes from the lysosomes in degradation of tumor cells in mammary gland carcinomas treated with cyclophosphamide or X irradiation has been described (1-3). Lysosomes increased in the tumor cells of treated animals, which suggested that lysosomal enzymes are involved in the autolysis of damaged tumor cell