In Schizosaccharomyces pombe, wee1 encodes a tyrosine kinase that inhibits entry into mitosis by phophorylating Cdc2, the universal cyclin-dependent kinase (Cdk) that regulates the G2/M transition in all eukaryotic cells. A search for suppressors of the G2 arrest caused by overexpression of weel led to the isolation of a new allele of swo1 (named swo1-w1), the gene coding for chaperone Hsp90, which is required to stabilise Weel. The swo1-w1 allele carries a glycine to aspartic acid substitution at amino acid 155 that results in a partial loss of Hsp90 function. Cells bearing the swo1-w1 mutation in combination with the point mutation cdc2-33 or cdc2-M26 showed severe mitotic defects. Genetic interactions were not observed in combination with point mutations in other cdc genes, suggesting that Cdc2 specifically interacts with Hsp90. This synthetic lethal swo1-w1 cdc2-33 (or cdc2-M26) strain had normal levels of Cdc2 protein and histone H1 phosphorylation activity, indicating that Hsp90 is required to enable Cdc2 to interact with its mitotic substrates or regulators, rather than for its proper folding or stabilisation. In a wild-type background, swo1-w1 mutant cells were sensitive to temperature as well as to other stress agents, such as KCI, ethanol and formamide. Under these stressful growth conditions, the swo1-w1 cells displayed anaphase B arrest and aberrant septation patterns, indicating that a subset of proteins involved in mitosis and cytokinesis is highly dependent on chaperone Hsp90 for function.