Cyclins play a key role in the induction of mitosis. In this paper we report the isolation of a cyclin A cDNA clone from Xenopus eggs. Its cognate mRNA encodes a protein that shows characteristic accumulation and destruction during mitotic cell cycles. The cyclin A polypeptide is associated with a protein that cross‐reacts with an antibody against the conserved ‘PSTAIR’ epitope of p34cdc2, and the cyclin A‐cdc2 complex exhibits protein kinase activity that oscillates with the cell cycle. This kinase activity rises more smoothly than that of the cyclin B‐cdc2 complexes and reaches a peak earlier in the cell cycle; indeed, cyclin A is destroyed before nuclear envelope breakdown. None of the cyclin‐cdc2 complexes show simple relationships between the concentration of the cyclin moiety and the kinase activity. All three cyclin associated kinases (A, B1 and B2) phosphorylate identical sites on histones with the consensus …