The 34 kilodalton protein product (p34) of the cdc2+ cell cycle control gene of Schizosaccharomyces pombe was expressed in bacteria. Monoclonal antibodies raised against this protein are capable of immunoprecipitating p34cdc2 from yeast lysates. Immunoprecipitates of [35S]methionine- and [32P]orthophosphate-labeled p34cdc2 were analyzed by two-dimensional gel electrophoresis. The cdc2+ gene product is homogeneous in size but resolves into seven species of differing charge. At least four of these species are phosphorylated. Phosphoamino acid analysis reveals that phosphorylation occurs mainly on threonine residues. The pattern of p34 phosphorylation is unaltered at the nonpermissive temperature in strains carrying temperature sensitive alleles of weel-50 and ran1-114 or in a strain overproducing the ran1+ gene product.