Interleukin 4 (IL-4) inhibited the differentiation of mouse myeloid leukemia M1 cells induced by leukemia inhibitory factor (LIF), interleukin 6, or dexamethasone and conversely enhanced the induction of M1 cell differentiation by 1 alpha,25-dihydroxyvitamin D3. IL-4 blocked LIF-induced differentiation of M1 cells when it was added to the culture medium within 10 h after LIF, but IL-4 did not block differentiation when it was added 12 h after LIF. These results indicate that IL-4 inhibited a critical intermediate step in myeloid leukemia cell differentiation. LIF markedly stimulated the expression of junB mRNA within 2 h but suppressed the expression of c-myb and c-myc after 2- and 12-h treatment, respectively. IL-4 did not significantly affect LIF-induced junB expression or suppression of c-myb expression. However, it interfered significantly with the LIF-induced suppression of c-myc gene expression. Similar results were obtained when interleukin 6 was used to induce differentiation of M1 cells. Dexamethasone and 1 alpha,25-dihydroxyvitamin D3 did not induce junB gene expression but suppressed the expression of c-myb and c-myc. IL-4 also interfered with dexamethasone-induced suppression of c-myc gene expression. On the other hand, IL-4 enhanced 1 alpha,25-dihydroxyvitamin D3-induced down-regulation of c-myc gene expression, consistent with its enhancement of differentiation. These results indicate that the change in c-myc expression induced by IL-4 in M1 cells is closely associated with the effect of IL-4 on the induction of differentiation of M1 cells.