The cdc2+ gene function has an important role in controlling the commitment of the fission yeast cell to the mitotic cycle and the timing of mitosis. We have raised antibodies against the cdc2+ protein using synthetic peptides and have demonstrated that it is a 34 kd phosphoprotein with protein kinase activity. The protein level and phosphorylation state remain unchanged during the mitotic cycle of rapidly growing cells. When cells cease to proliferate and arrest in G1 the protein becomes dephosphorylated and loses protein kinase activity. Exit from the mitotic cycle and entry into stationary phase may be controlled in part by modulation of the cdc2 protein kinase activity by changes in its phosphorylation state.