The G2-M-phase transition is controlled by cell-cycle checkpoint pathways which inhibit mitosis if previous events are incomplete or if the DNA is damaged. Genetic analyses in yeast have defined two related, but distinct, pathways which prevent mitosis--one which acts when S phase is inhibited, and one which acts when the DNA is damaged. In the fission yeast Schizosaccharomyces pombe, many of the gene products involved have been identified. Six 'radiation checkpoint' (rad) gene products are required for both the S-M and DNA-damage checkpoints, whereas Chk1, a putative protein kinase, is required only for the DNA-damage checkpoint and not for the S-M checkpoint following the inhibition of DNA synthesis. We have genetically defined a third mitotic control checkpoint pathway in fission yeast which prevents mitosis when passage through 'start' (the commitment point in G1) is compromized. In cycling cells arrested at start, mitosis is prevented by a Chk1-dependent pathway. In the absence of Chk1, G1 cells attempt an abortive mitosis with a 1C DNA content without entering S phase. Similar results are seen in the absence of Rad17, a typical example of a rad gene product. Genetic dissection of checkpoints in logarithmically growing fission yeast has identified a pathway that couples mitosis to correct passage through start. This pathway is related to the DNA-structure check-points which ensure that mitosis is dependent on the completion of replication and the integrity of the DNA. We propose that all three mitotic control checkpoints monitor distinct DNA or protein structures at different stages in the cell cycle.