Most reports have now described two populations of childhood ALL patients: those with thymic (T) cell receptors and those lacking receptors on their neoplastic cells. Assays for the surface receptors of the T and thymic-independent (B) system were used to study forty-seven patients with ALL whose bone marrow contained a mean of 85% leukaemic cells. Two patients had T-cell disease and thirty-six were non-T and non-B. nine patients were identified whose leukaemic cells had membrane properties associated with the B-cell system: surface immunoglobulin, Fc receptors and/or complement receptors. Combined T and B receptors were found in one case. The same surface characteristics were found on leukaemic cells from these patients' bone marrow, blood, pleural and cerebrospinal fluid. Studies showed that the leukaemic cells were not of monocytic or granulocytic origin. Although a remission was obtained in each patient, the relapse rate of the B-cell group was worse than a similarly treated group of thirty-six non-T, non-B ALL patients (P less than 0.001). Initial total leucocyte counts of the B-cell group were greater than the non-T, non-B group (P 0.05), but when the patients in both groups with total leucocyte counts greater than 25,000/mm3 were compared, the relapse rate of the B-cell patients was significantly worse (P less than 0.025). The results show that patients with leukaemic cells possessing B-cell properties comprise a significant proportion of ALL cases, and their presence on leukaemic cells has an ominous significance.