A plasmid, pHR402, containing SV40 sequences that include a truncated early region bearing an intact t-coding sequence and a functionally intact late region, was introduced into thymidine kinase deficient (tk-) mouse L cells by cotransformation with a cloned tk gene. tk+ cotransformants synthesized SV40 t but not T antigen, and no truncated T-coding sequence products were detected. The viral sequences of pHR402 were reconstituted as a virus in COS1 cells, and acute infection of untransformed mouse cells with this viral stock (SV402) also led to the appearance of t but not T or a truncated T. Abortive transformation assays of such infected cells were negative, as were those performed on the same cells infected with either of two viral mutants (dl883 and dl884), each of which leads to T but not t synthesis. However, mixed infection with SV402 and either dl883 or dl884 led to a clear abortive and permanent transformation response. Thus, at least in part, t and T appear to function in a complementary fashion in eliciting transformation expression by SV40-infected cells.