In an attempt to determine the function of the intron, we have generated a deletion mutant lacking precisely the intron of SV40 16S mRNA. This SV40 deletion mutant produced no stable late viral RNA, which suggested a defect is post-transcriptional processing of the viral RNA. Subsequently, we used this mutant as the recipient for the insertion of an isolated mouse beta maj globin intron. Cloned recombinants harboring the inserted intron in the sense direction give rise to stable mRNA. These observations suggested that introns represent functional elements in the generation of certain mRNAs. On the other hand, the recent detection of a number of eukaryotic genes which are uninterrupted by intervening sequences, as well as certain late mutants of SV40, which appear to produce unspliced 19S late mRNAs, have suggested that alternative mechanisms must exist for the expression of nonspliced transcripts.