We have used antiserum raised against a beta-galactosidase-cdc10+ fusion protein to identify the protein product of the cdc10+ start gene of Schizosaccharomyces pombe. This gene is required for progress through the G1 phase of the cell cycle and for activating processes such as the increase in histone mRNA level in preparation for S phase. The protein has an apparent molecular weight of 87,000 and is phosphorylated on multiple serine residues. The protein remains phosphorylated throughout the mitotic cell cycle and shows no significant steady-state changes in level. The antiserum has also detected a protein similar in size to p87cdc10 in human cells.