Cyclins play an important role in cell cycle regulation. At least five classes of cyclins have been identified--A, B, C, D, and E. B cyclins are generally of two types in most organisms--B1 and B2. We have mapped the gene for human cyclin B1 (CCNB1) to human chromosome 5 (region q13-qter) by Southern blot analysis of human x Chinese hamster somatic cell hybrid panels. Many more cyclin B-related sequences have been identified in the mouse (Cycb-1 to Cycb-10) and have been mapped to chromosomes 4, 5, 7, 8, 13, 14, 15, and 17. Based on our mapping of human CCNB1 and known evolutionary conservation of chromosomal regions, we propose that the homologous cyclin B1 locus, Cycb-4, on mouse chromosome 13 is a functional gene.