We have used the human leukemia cell line K562 as a model to study the role of c-myc in differentiation and apoptosis. We have generated stable transfectants of K562 constitutively expressing two c-Myc inhibitory mutants: D106-143, that carries a deletion in the transactivation domain of the protein, and In373, that carries an insertion in the DNA-interacting region. We show here that In373 is able to compete with c-Myc for Max binding and to inhibit the transformation activity of c-Myc. K562 cells can differentiate towards erythroid or myelomonocytic lineages. K562 transfected with c-myc mutants showed a higher expression of erythroid differentiation markers, without any detectable effects in the myelomonocytic differentiation. We also transfected K562 cells with a zinc-inducible max gene. Ectopic Max overexpression resulted in an increased erythroid differentiation, thus reproducing the effects of c-myc inhibitory mutants. We also studied the role of c-myc mutants and max in apoptosis of K562 induced by okadaic acid, a protein phosphatases inhibitor. The expression of D106-143 and In373 c-myc mutants and the overexpression of max reduced the apoptosis mediated by okadaic acid. The common biochemical activity of D106-143 and In373 is to bind Max and hence to titrate out c-Myc to form non-functional Myc/Max dimers. Similarly, Max overexpression would decrease the relative levels of c-Myc/Max with respect to Max/Max. The results support a model where a threshold of functional c-Myc/Max is required to maintain K562 cells in an undifferentiated state and to undergo drug-mediated apoptosis.