Mouse thymidine kinase-negative L cells were transformed with a cloned rabbit chromosomal beta-globin gene linked to the clone thymidine kinase gene of herpes simplex virus type 1. Most thymidine kinase-positive cell lines contained one or more copies of rabbit beta-globin DNA and produced up to 2,000 copies of rabbit beta-globin RNA per cell indistinguishable from its authentic counterpart. No mouse beta-globin mRNA was detected.