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The DNA synthesis, the cytolytic activity, and the ultrastructure of cytolytic T lymphocytes (CTL) derived from mixed mouse thymocyte culture on day 5 were studied. CTL of thymic origin were absorbed by centrifugation on the surface of target cell (TC) monolayers. At different time intervals after absorption, single tubular structures (TS) and complex of tubular structures (CTS) linked with ergastoplasmic reticulum, secretory granules, Golgi apparatus, coated vesicles, and multivesicular bodies were detected in the CTL cytoplasm. The linkage of CTS with ergastoplasmic reticulum, ribosomes, and secretory lymphocyte mechanism suggests the possibility of a secretory-receptor mechanism of TC cytolysis. The release of numerous secretory vacuoles was accompanied by the enlargement of the cytoplasmatic lymphocyte membrane surface, which seemed to cause its shedding. 'Membranosomas' were observed on CTL membrane; their role is still obscure and awaits further study.