cdc2+ and CDC28 play central roles in the cell division cycles of the widely divergent yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. The genes encode protein kinases that show 62% protein sequence identity and are capable of cross-complementation. Monoclonal antibodies were raised against p34cdc2, and a subset recognize p36cdc28. The cross-reacting antibodies detected a 34 kd homolog of the p34cdc2/p36CDC28, protein in HeLa cells. Human p34 was also recognized by an affinity-purified polyclonal anti-p34cdc2 serum. Peptide mapping of p34cdc2, p36CDC28, and human p34 revealed complete conservation of four tryptophan residues in the three proteins. p34 thus appears to be closely related to the two yeast proteins. In addition, a p34 immune complex showed protein kinase activity in vitro, and HeLa cell p34 interacts with p13, the human homolog of the suc1+ gene product of S. pombe.