In a genetic screen for second-site mutations that are lethal in combination with a deletion of the amino terminus of histone H3, we have uncovered three new gene products that regulate the Saccharomyces cerevisiae Swel kinase. The Swel protein kinase phosphorylates tyrosine residue 19 of Cdc28 and inhibits its activity. One histone synthetic-lethal gene, HSLl, encodes a putative protein kinase that has high sequence and functional homology to fission yeast cdrl/niml, an inhibitory kinase of weel. Another gene, HSLl, is a novel negative regulator of Swel function. Sequences similar to Hsl7 exist in Caenorhabditis elegans and humans. In addition, we have isolated a dosage-dependent suppressor, OSSl, of hsU and hsl7. OSSl is important for the transcriptional repression of SWEl and CLN2 in G2. Mutations in HSLl and HSL7 therefore cause hyperactivity of the Swel kinase, which in turn decreases mitotic Cdc28 kinase activity. Moreover, HSL5 is identical to CDC28, further suggesting that it is the decreased Cdc28 kinase activity in these hsl mutants that causes lethality in the histone mutant background. Because neither HSLl nor HSL7 is essential in yeast, and histone transcription is unaffected by the hsl5/cdc28 mutation, it is unlikely that synthetic lethality results from reduced transcription of HSLl and HSL7 caused by histone mutations, or from reduced histone transcription when Cdc28 kinase activity is compromised. We suggest that these cell cycle regulators function in a pathway upstream of both histones H3 and H4, thereby modulating histone function in the cell cycle.