We present evidence that mammalian cells have the ability to generate functional mRNA molecules by trans‐splicing. Rat cells, transformed by an early SV40 DNA fragment (Bst/Bam) synthesize a truncated T antigen (T1 antigen), although the cells do not have a direct sequence homology for the T1 antigen at the DNA level. The Bst/Bam DNA fragment encodes exclusively for the second SV40 T antigen exon (aa 83‐708) and contains the entire small t antigen intron. To synthesize the corresponding mRNA (T1 mRNA), the cells utilize a cryptic 5′ splice site within the second exon (codons for aa 131/132) as donor site and the upstream small t antigen 3′ splice site as the acceptor site. Since these sites are in an inverted order on the pre‐mRNA, two Bst/Bam transcripts are required to generate one T1 mRNA molecule. HeLa cell nuclear extracts also performed the trans‐splicing reaction in vitro.