This study describes the opposing effects that interleukin (IL) 4 exerts on the B cell stimulatory factor (BSF-MP6) and IL 2-dependent proliferation and differentiation of cells of one selected B-type chronic lymphocytic leukemia cell clone (I83), which depend on the nature of the activation inducer. In I83 cells activated by a 1-h pulse of 12-O-tetradecanoylphorbol 13-acetate, the BSF-MP6-dependent DNA synthesis was strongly enhanced by 50-100 U/ml of recombinant IL 4. Recombinant IL 2 stimulation was necessary only when a suboptimal dose of BSF-MP6 was used. The differentiation was also markedly enhanced by IL 4 as measured by quantitation of IgM secretion both at the population (enzyme-linked immunosorbent assay analyses of the supernatant) and single-cell level (enzyme-linked immunospot technique), by morphological examination of the maturation stage and flow cytometric analysis of differentiation-associated surface antigens (CD11c, FMC7, PCA-1 and CD38). No Ig isotype switch was found. In contrast, DNA synthesis and differentiation of I83 cells, activated by Staphylococcus aureus Cowan strain I (SAC) and co-stimulated with BSF-MP6 plus IL 2, were strongly inhibited by IL 4, both when it was added simultaneously with SAC or after 2 days of SAC exposure. Analysis of the cell-cycle progression of SAC and BSF-MP6 plus IL 2 and IL 4-stimulated cells by acridine orange staining and fluorescence-activated cell sorter (FACS) analysis demonstrated an arrest of a minor cell population in G0 and a block of the transition of G1 cells to S phase. Neither the enhancing nor the inhibitory effect of IL 4 on the proliferation and differentiation of I83 cells was an indirect effect via IL 4-induced activation of contaminating T cells, monocytes or natural killer cells, as shown by experiments where these cell types were depleted by FACS sorting. Furthermore the expression of CD23 and CD25 was not inhibited by IL 4. The results thus demonstrate contrasting biological effects of IL 4 on clonal leukemic B cells depending on the nature of the activation and progression stimuli. This adds to the emerging picture of a very complex cytokine and cell-to-cell contact-mediated regulation of the activation and subsequent growth and/or differentiation of human B cells.