Genetic analysis in budding yeast has shown that multiple G1 cyclins and cyclin-dependent kinases control cell cycle entry, polarized growth, and spindle pole duplication. The G1 cyclins Cln1 and Cln2 associate with the cyclin-dependent kinase Cdc28 to facilitate cell cycle progression and development of the cleavage apparatus. We have developed a chemical genetic approach toward the discovery of compounds that target G1 control pathways by screening for compounds that selectively kill a yeast strain lacking the G1 cyclins Cln1 and Cln2. A class of small molecules was identified that is highly toxic toward the cln1 Delta cln2 Delta double mutant and has relatively little effect on wild-type yeast. We call these compounds 'clinostatins' for their selectivity toward the cln1/2 deletion strain. Clinostatins were used in a genome-wide chemical synthetic lethality screen to identify other genes required for growth in the presence of the drug. Other deletions that were sensitive to the drug include members of the protein kinase C(PKC)-dependent MAP kinase pathway. These results suggest an approach for combining chemical synthetic lethality and chemical genomic screens to uncover novel genetic interactions that can be applied to other eukaryotic pathways of interest.