The nimA and bimE genes of Aspergillus nidulans respectively encode a 79 kDa protein kinase that is a positive regulator of mitosis and a 229 kDa protein that is a negative regulator of mitosis. Either overproduction of nimA or inactivation of bimE can induce mitosis and override the checkpoint associated with incomplete DNA replication. Double mutants between temperature-sensitive nimA and bimE alleles undergo chromatin condensation and spindle polymerization at restrictive temperature, suggesting that the p79nimA kinase is not required for chromatin condensation and spindle polymerization when bimE function is defective. In contrast double mutants carrying ts bimE and nimEcyclinB mutations or bimE and nimTcdc25 mutations are blocked in interphase at restrictive temperature. These results indicate that the mitotic block caused by inactivation of bimE requires activation of the p34cdc2/MPF kinase for chromatin condensation and spindle polymerization to occur. Antibodies against bimE fusion proteins have been used to study p229bimE in wild-type cells and cells overexpressing the bimE gene.