The expression of c-myb proteins in transformed and normal murine cells was investigated. Two c-myb proteins, p75c-myb and p90c-myb, were detected in normal thymocytes and cell lines with intact c-myb genes. These most likely differ by the inclusion of additional amino acids encoded by an alternatively spliced c-myb mRNA. The use of this alternative exon is therefore not a feature exclusively of those cells with viral integrations in their c-myb gene. Smaller c-myb proteins in myeloid leukemic cell lines with rearranged c-myb genes were also characterized. Viral integration into the 5' region of the c-myb gene in the W265 and W274 cell lines leads to the synthesis in each case of two amino-terminally truncated c-myb proteins. By contrast, in NFS60 cells, viral integration into a more 3' region of the c-myb locus (but upstream of an alternate exon) leads to the production of a single p50c-myb protein.