A chicken calmodulin (CaM) gene has been expressed in mouse C127 cells using a bovine papilloma virus (BPV)-based vector (BPV-CM). The vector-borne genes produce a mature mRNA of the expected size that is present on cytoplasmic polyribosomes. In clonal cell lines transformed by BPV-CM, expression of the CaM gene produced CaM levels 2- to 4-fold above those observed in cells transformed by BPV alone. Increased intracellular CaM caused a reduction of cell cycle length that is solely due to a reduction in the length of the G1 phase. A comparison of six cell lines revealed a linear relationship between the intracellular CaM concentration and the rate of G1 progression. These data provide the first evidence that specific elevation of CaM levels directly affects the rate of cell proliferation.