Expression of the class I transplantation antigens of the major histocompatibility complex (MHC) is suppressed in cells transformed by the oncogenic human adenovirus 12 (Ad12). This suppression of class I antigen expression, which contributes to the tumorigenic phenotype of the transformed cells, has also been observed in some naturally occurring cancers. In the present study, the rate of transcription initiation of class I genes was measured by a nuclear run-on assay in Ad5- and Ad12-transformed cells of three different types. The rate of transcription was the same in all three. The stability of the class I messenger RNA was also examined and found to be the same in all three cell types. The results indicate that in Ad12-transformed cells the suppression is caused by an inhibition of the post-transcriptional processing of class I MHC messenger RNA in the nucleus.