We have used immunofluorescence staining to study the subcellular distribution of cyclin A and B1 during the somatic cell cycle. In both primary human fibroblasts and in epithelial tumor cells, we find that cyclin A is predominantly nuclear from S phase onwards. Cyclin A may associated with condensing chromosomes in prophase, but is not associated with condensed chromosomes in metaphase. By contrast, cyclin B1 accumulates in the cytoplasm of interphase cells and only enters the nucleus at the beginning of mitosis, before nuclear lamina breakdown. In mitotic cells, cyclin B1 associates with condensed chromosomes in prophase and metaphase, and with the mitotic apparatus. Cyclin A is degraded during metaphase and cyclin B1 is precipitously destroyed at the metaphase----anaphase transition. Cell fractionation and immunoprecipitation studies showed that both cyclin A and cyclin B1 are associated with PSTAIRE-containing proteins. The nuclear, but not the cytoplasmic form, of cyclin A is associated with a 33-kD PSTAIRE-containing protein. Cyclin B1 is associated with p34cdc2 in the cytoplasm. Thus we propose that the different localization of cyclin A and cyclin B1 in the cell cycle could be the means by which the two types of mitotic cyclin confer substrate specificity upon their associated PSTAIRE-containing protein kinase subunit.