Exposure of human leukemic lymphoblasts in suspension cultures to low concentrations of vinblastine and vincristine results in alterations in cell shape and leads to the formation and release of a large number of membrane-lined vesicles from the cytoplasm. Separation of these vesicles from peripheral cytoplasm is effected through alignment and fusion of small vacoules. Similar vesicle formation is seen neither in fibroblasts exposed to vinblastine nor in lymphoblasts exposed to bleomycin or adriamycin. Possible relation of this phenomenon to vinblastine- and vincristine-induced cytotoxicity, spherocytosis, and thrombocytosis is discussed.