Summary cdc2+ and CDC28 play central roles in th& cell division cycles of the widely divergent yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. The genes encode protein klnases that show 62% protein sequence identity and are capable of cross-complementation. Monoclonal antibodies were raised against p34cdc2, and a subset tecognize~ 36~~~~~. The cross-reacting antibodies detected a 34 kd homolog of the p34cdc2/p36cDc28, protein in HeLa cells. Human~ 34 was also recognized by an affinity-purified polyclonal anti-p34cdcr serum. Peptide mapping of p34cdc2, p36cDc28, and human~ 34 revealed complete conservation of four tryptophan residues in the three proteins.~ 34 thus appears to be closely related to the two yeast proteins. In addition, a~ 34 immune complex showed protein kinase activity in vitro, and HeLa cell~ 34 interacts with~ 13, the human homolog of the sucf+ gene product of S. pombe.