Sequential immunological and cytokinetic studies were done on individual lymphoid cells in the peripheral blood of 10 children with previously untreated acute lymphoid leukemia (8 non-T--non-B, 1 T, and 1 B-ALL) and on lymphoid bone marrow cells (4 non-T--non-B ALL). The results obtained suggest that some, but not all, of the children considered to have non-T--non-B ALL had proliferating leukemic cells with surface immunoglobulin (Sig) in the peripheral blood. The absolute number of these SIg+ proliferating leukemic cells, however, was always smaller than the absolute number of SIg- proliferating leukemic cells. A similar observation was made in the child with B-ALL. On the other hand, in the blood of the child with T-ALL, no SIg+ proliferating leukemic cells were detected. The results indicate that sequential analyses of different parameters on single cells might permit a more precise investigation of relations between cell proliferation and cell differentiation or of interactions between normal and neoplastic lymphoid cells.