The protein Max binds to c-Myc and the heterodimer c-Myc/Max seems to be the active form in vivo. While the expression of c-myc is extensively regulated, no major changes in max expression have been reported so far with respect to differentiation. We have studied the expression of c-Myc and Max during in vitro differentiation of the bipotent human myeloid leukemia K562 cell line. This cell model system allowed us to compare c-Myc and Max expression during differentiation along erythroid (induced by 1-beta-D-arabinofuranosyl-cytosine) and myelomonocytic lineages (induced by 12-0-tetradecanoylphorbol-13-acetate). We found that c-myc expression decreased as a result of both differentiating treatments. The expression level of max remained unchanged during myelomonocytic differentiation. In contrast, max mRNA and protein were dramatically down-regulated during erythroid differentiation of K562 cells, thus demonstrating that max gene is subjected to regulation during differentiation. We also studied the expression of the other two described members of the c-Myc network: mxi1 and mad. mxi1 expression increased during erythroid differentiation but was strongly down-regulated during myelomonocytic differentiation of K562. mad was constitutively expressed during K562 erythroid differentiation and slightly increased during induction of the myelomonocytic pathway. We have obtained K562 sublines stably transfected with a zinc-inducible c-myc gene. In these clones the overexpression of c-Myc did not interfere with TPA-induced myelomonocytic differentiation. In contrast, erythroid differentiation was significantly inhibited upon c-myc induction despite the down-regulation of endogenous max expression. These results suggest a differential role for c-Myc in the human myeloid cell differentiation depending on the cell lineage.