We have isolated a Leishmania mexicana homologue of the fission yeast suc1 gene using PCR with oligonucleotides designed to conserved regions of cdc2 kinase subunits (cks). The product of cks1 is a 12 kDa polypeptide, which has 70% identity with human p9cks1 and 44% identity with fission yeast p13suc1.p12cks1 was detected in the three life-cycle stages of L. mexicana by immunoblotting. Recombinant p12cks1 (p12cks1his) bound to agarose beads was used as a matrix to affinity-select histone H1 kinase complexes from Leishmania, yeast and bovine extracts. Immunoblotting showed that yeast and bovine cdc2 kinase bound to p12cks1his, thus demonstrating functional homology between L. mexicana p12cks1 and yeast p13suc1. Histone H1 kinase activity was found at a high level in the proliferative promastigote and amastigote forms of L. mexicana, but at a low level in the non-dividing metacyclic form. These activities are likely to be the same as the leishmanial p13suc1 binding kinase (SBCRK) described previously [Mottram, Kinnaird, Shiels, Tait and Barry (1993) J. Biol. Chem. 268, 21044-21051]. A distinct cdc2-related kinase, L. mexicana CRK1, was also found to associate with p12cks1his but affinity-depletion experiments showed that CRK1 was not responsible for the histone H1 kinase activity associating with p12cks1his in promastigote cell extracts. The finding that p12cks1 associates with at least two cdc2-related kinases, SBCRK and CRK1, is consistent with the presence of a large gene family of cdc2-related kinases in trypanosomatids, a situation thought to be more similar to higher eukaryotes than yeast.