Mouse thymidine-kinase-negative L cells were transformed with a cloned chicken ovalbumin gene linked to the cloned thymidine kinase gene from herpes simplex virus type 1. Most thymidine-kinase-positive clones contained one or more copies of the ovalbumin gene, which were stably maintained for at least 6 months during continuous culture under selective conditions. Transcription of the ovalbumin gene was detected at a low level, producing RNA molecules that were correctly spliced and polyadenylylated by comparison with genuine ovalbumin mRNA. However, the 5' end of these RNA molecules does not correspond to that of ovalbumin mRNA.