We investigated the effects of recombinant human interleukins 1 to 6 (rIL-1 to -6) on the proliferation of blast cells from patients with acute lymphoblastic leukemia (ALL). The 3H-TdR incorporation in the presence of various cytokines was examined in cells from 14 patients: 12 with B-lineage ALL, 1 with T-lineage ALL and 1 with biphenotypic leukemia. In B-lineage ALL, a significant increase in 3H-TdR incorporation was observed in 5/12 cases (42%) in the presence of rIL-1 alpha, in 10/12 cases (83%) with rIL-2, in 9/12 cases (75%) with rIL-3, in 3/6 cases (50%) with rIL-4, in 4/6 cases (67%) with rIL-5, and in 4/12 cases (33%) with rIL-6. The mean stimulation index of the cells showing a positive response was 1.74 for rIL-1 alpha, 3.40 for rIL-2, 2.55 for rIL-3, 1.86 for IL-4, 1.56 for rIL-5, and 2.97 for rIL-6. T-lineage ALL cells were stimulated only in the presence of rIL-2, and biphenotypic leukemia cells were not stimulated by any of the cytokines tested.