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 Swe1 kinase. The Swe1 protein kinase phosphorylates tyrosine residue 19 of Cdc28 and inhibits its activity. One histone synthetic-lethal gene, HSL1, encodes a putative protein kinase that has high sequence and functional homology to fission yeast cdr1/nim1, an inhibitory kinase of wee1. Another gene, HSL7, is a novel negative regulator of Swe1 function. Sequences similar to Hsl7 exist in Caenorhabditis elegans and humans. In addition, we have isolated a dosage-dependent suppressor, OSS1, of hsl1 and hsl7. OSS1 is important for the transcriptional repression of SWE1 and CLN2 in G2. Mutations in HSL1 and HSL7 therefore cause hyperactivity of the Swe1 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 HSL1 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 HSL1 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.