A hybrid gene containing the 5'' sequence of a hamster histone H3 gene and the coding sequence of the bacterial neomycin-resisance gene (neo) was constructed. Upon transfection into the hamster fibroblasts cell line K12, the hybrid gene exhibited cell cycle-dependent regulation, as evidence by the maximal accumulation of the neo transcripts during synthesis of DNA in the cell cycle. In addition, cells arrested in the prereplicative phase, as a consequence of the K12 temperature-sensitive mutation, produced significantly less neo messenger RNA.