Friend murine leukemia cells induced to undergo erythrocytic differentiation by dimethyl sulfoxide give rise to progeny resembling ortho- or polychromatic normoblasts, which usually do not complete the maturation process to yield forms analogous to erythrocytes. Treatment of these differentiated cells with cytochalasin B can lead to a high proportion (i.e., 80-85%) of enucleated cells in vitro. Nuclear extrusion in cells induced to differentiate by dimethyl sulfoxide and subsequently treated with cytochalasin B began within 24-36 hr of exposure to the antibiotic, with the appearance of a pre-enucleated stage in which the cell nucleus became pycnotic and eccentrically located. Maximum enucleation occurred after 72-96 hr of exposure to cytochalasin B and was significantly enhanced when dimethyl sulfoxide was included in the culture medium during the period of treatment with cytochalasin B. Enucleation appeared to take place only in differentiated progeny, because nondifferentiated cells treated with cytochalasin B alone yielded a population of multinucleated cells. The findings indicate that highly tumorigenic nondifferentiated Friend erythroleukemia cells can be converted in high yield to mature enucleated forms that are unable to proliferate in vitro.