In order to explore their differentiation capacity peripheral blasts from 4 children with acute lymphoblastic leukemia of the common type (cALL) as well as cells of the Reh line were cultured in diffusion chambers implanted intraperitoneally into preirradiated CBA mice (8 gy). At various times after initiation of the culture, changes in the membrane marker profile were studied by investigation of surface antigens and rosetting phenomena. In the course of the culture, the cells of two patients, T. H. and I.G., developed surface immunoglobulin (sIg) in combination with the CALL antigen (cALLA). Simultaneously, the cells of I.G. containing cytoplasmic Ig prior to cultivation acquired a receptor for mouse erythrocytes (EM). Besides quite a few cells developing the T-cell antigen, the majority of the cell yield in patient V.M. expressed sIg alone or together with the cALLA. Comparable results together with an increasing portion of mouse rosette forming cells were obtained in the fourth patient, C.M. In case of the Reh-line the cells developed the T-cell antigen, and in one of two experiments they further acquired a receptor for sheep erythrocytes. conversely, in a second experiment and EM-receptor was detected. Our data suggest that leukemic blast cells carrying the cALLA are representative of an early developmental stage of lymphatic ontogeny.