Rous sarcoma virus was shown to induce in chicken embryo fibroblasts (CEF) a 4.1-kilobase mRNA (designated CEF-147) encoding a 603-amino acid protein. Analysis of the protein sequence showed that it shared 59% amino acid identity with sheep prostaglandin G/H synthase, the enzyme that catalyzes the rate-limiting steps in the production of prostaglandins. Significant differences, at both the protein and mRNA levels, existed between the src oncogene product-inducible prostaglandin synthase and the protein isolated and cloned from sheep seminal vesicle, suggesting that the src-inducible prostaglandin synthase may be a new form of the enzyme. A distinguishing feature of src-inducible prostaglandin synthase mRNA is its low abundance in nonproliferating chicken embryo fibroblasts and its relatively high abundance in src-transformed cells. Additionally, the majority of the src-inducible prostaglandin synthase RNA present in nonproliferating cells was found to be nonfunctional because of the presence of an unspliced intron that separated the signal peptide from the remainder of the protein. Upon mitogenic stimulation, this intron was removed, resulting in the induction of fully-spliced CEF-147 mRNA.