We have used mRNA differential display to search for cellular genes whose expression is modulated in SV40-transformed cells. We identified a gene, the α1 subunit of type XI procollagen (α1 (XI)), whose mRNA level is reduced in most F111 rat cell lines that express either wild-type SV40 T antigen or an amino-terminal fragment of T antigen (T147D). α1(XI) mRNA expression is also reduced in rat embryo fibroblasts transformed by SV40. This phenomenon is not unique to rat cells, because α1(XI) mRNA expression is repressed in C3H10T1/2 mouse cells transformed by either T147D or wild-type SV40. Interestingly, α1 (XI) mRNA levels are not reduced in F111 cells that express only the polyomavirus large T antigen, but are reduced in F111 cells transformed by polyoma virus (which expresses all three polyomavirus T antigens) and in F111 cells that express the adenovirus E1A proteins. Thus, repression of α1(XI) mRNA expression may be a common feature of transformation by DNA tumor viruses.