The Max gene encodes a protein that interacts specifically with the Myc protein to form a heterodimer with high affinity for the specific cognate DNA binding site of Myc. Here we examine the expression of Max RNA in comparison to Myc RNA during cell growth and differentiation. Two species of RNA, a major 2.0- and a minor 1.7-kilobase species, hybridized specifically to a Max cDNA probe in all human and murine cell lines that were tested. Unlike Myc, the steady-state level of Max RNA is not significantly modulated with respect to proliferation or differentiation. Max RNA is expressed in quiescent BALB/c 3T3 cells and is modestly increased 3 h after addition of serum to the quiescent cells. In contrast to Myc RNA, Max RNA does not decline immediately upon induction of differentiation of HL60 cells by dimethyl sulfoxide, and only a modest decrease of Max RNA was observed 72 h after induction of differentiation. Unlike Myc RNA, Max RNA is relatively stable with a half-life of greater than 3 h and, therefore, does not exhibit the characteristic short half-life of RNAs encoded by most immediate early genes. The human Max gene was localized to chromosome 14, band q23. With respect to the recurring abnormalities in human tumors, this region of chromosome 14 is involved in deletions in B-cell chronic lymphocytic leukemia and malignant lymphomas and in the 12;14 translocation in uterine leiomyomas.