In the interest of identifying components of the Cdc28 protein kinase complex, dosage suppression analysis was performed on temperature-sensitive and dominant negative CDC28 mutations. Dosage suppression is based on a rationale in which elevated expression of wild-type genes can rescue mutations in a target gene as a result of interaction between the respective encoded proteins. Three sequences capable of rescuing a temperature sensitive cdc28 mutation were isolated from a library of wild-type genomic DNA segments in the high copy vector YEp13. Two of these, named CLN1 and CLN2 were found to encode closely related proteins with homology to cyclins. The third, CKSI, encodes an 18K (K = 103Mr) protein that has been shown to be a component of the Cdc28 protein kinase complex and is a homolog of the suc1+ product of fission yeast. A number of dosage suppressors of the CDC28-dn1 dominant negative mutation have been isolated. The one analyzed to date encodes a truncated subunit of the mitochondria1 enzyme succinyl-CoA synthetase. The basis for suppression in this case remains to be elucidated.