suc1 is an essential gene initially identified for its ability to rescue certain temperature-sensitive alleles of cdc2 in Schizosaccharomyces pombe. The role of suc1 in the regulation of the cdc2 kinase is not well understood. In our study, we have characterized the biochemical effect of loss of suc1 function on specific cdc2-cyclin complexes. We show that the cig1 cyclin is associated with cdc2 and that the cdc2-cig1 kinase is activated at mitosis, with kinetics similar to those of the cdc2-cdc13 kinase. We provide evidence that loss of suc1 function affects the kinase activity of the two distinct mitotic forms of the cdc2 kinase. We also show that a dramatic increase in the level of the cdc13 protein is associated with loss of suc1. These results suggest that mitosis cannot be properly completed in the absence of suc1, possibly because of an increase in the level of cdc2-cdc13 complex, and support the idea of a role for suc1 in the regulation of multiple forms of the cdc2 kinase.