MH2 and MC29 are highly related myc-containing avian retroviruses. We found that MH2, unlike MC29, synthesizes a 2.6-kilobase subgenomic mRNA containing myc sequences as well as sequences from the 5' end of the genome. A 57-kilodalton protein containing myc, but not gag, sequences (p57myc) was detected by hybrid selection and in vitro translation of RNA from MH2-transformed cells. Gradient separation of MH2 intracellular RNAs indicated that p57myc is encoded by the subgenomic RNA. A highly oncogenic MH2 virus variant (MH2YS3) (M. Linial, Virology 119:382-391, 1982) was shown to encode only p57myc and not P100, the previously described MH2-encoded polyprotein (Hu et al., Virology, 89:162-178, 1978). Cells transformed by subclones of this virus synthesized predominantly the 2.6-kilobase RNA rather than genomic 5.4-kilobase RNA. These results suggest that only p57myc is required for maintenance of the transformed state after MH2 infection.