CYCLINS are proteins synthesized during each cell cycle and abruptly destroyed in each mitosis. Cyclins have been implicated in the induction of mitosis and are associated with the serine-threonine protein kinase p34cdc2 as components of mitosis promoting factor (MPF). On the basis of conserved sequence motifs cyclins can be divided into A or B types. We recently cloned a human cyclin B and showed that cyclin B expression is regulated transcriptionally and post-translationally during the cell cycle, and that cyclin B associates with p34cdc2. Here we report that human cyclin A messenger RNA and protein levels also vary during the cell cycle, and increase and decrease in advance of cyclin B levels. Cyclin A is associated with a protein of relative molecular mass 33,000 that is related to, but distinct from, p34cdc2, and this complex has histone H1 kinase activity in vitro. Cyclin A is identical to p60, a protein that associates with p34cdc2 in interphase cells and with adenovirus E1A in transformed cells.