Entry into mitosis in the fission yeast Schizosaccharomyces pombe involves the interaction of a number of genes with the major cell cycle control gene, cdc2+. One of these, cdc13+, encodes a protein with homology to cyclin. By indirect immunofluorescence microscopy using antibodies to the appropriate bacterially-expressed protein, we have shown that both cdc13 and cdc2 are nuclear proteins in S. pombe. Both are localised to a nuclear domain distinct from that occupied by the DAPI-staining chromatin. The immunofluorescence signals of both proteins show a progressive increase during interphase but are undetectable at mitosis. Loss of cdc13 fluorescence at mitosis reflects the destruction of the protein. Thus, it behaves as a classic cyclin. This is not the case for cdc2, the level of which remains constant through the cell cycle. Cells carrying a disrupted copy of the cdc13+ gene fail to accumulate either cdc13 or cdc2 in the nucleus. Cells carrying a disrupted cdc2+ gene fail to accumulate cdc2 but reveal apparently normal levels of cdc13. cdc13 therefore appears to be required to localise cdc2 to the nucleus but not vice versa. The destruction of cdc13 at mitosis may allow cdc2 to redistribute to the cytoplasm.