The max gene encodes a heterodimeric partner of Myc. We have recently identified an alternative max mRNA (delta max) that contains an additional internal exon introducing an in-frame translational termination. Here we have studied the expression of human max mRNAs by Northern blotting analysis. In addition to the major 2.3-kb mRNA form, four bands were identified. Our results indicate that these bands represent differentially spliced mRNA forms, which contain altogether three open reading frames. In addition to the previously identified Max and delta Max proteins, sequence analysis of a 3.5-kb mRNA form predicted a protein that resembles delta Max in structure. Like delta Max, this protein enhanced the number of transformed foci in the ras-myc co-transformation assay. Although the 3.5-kb mRNA represents a minor form in actively proliferating cells, a shift from the major 2.3-kb mRNA to the 3.5-kb form was observed in response to high cell density or acidification of the growth medium. Our results indicate the presence of several differentially spliced mRNA forms of the max gene, and suggest a possible mechanism for the production of functionally distinct Max proteins.