Antigen receptor genes are assembled by site-specific DNA rearrangement. The recombination activator genes RAG-1 and RAG-2are essential for this process, termed V(D)J rearrangement. The activity and stability of the RAG-2 protein have now been shown to be regulated by phosphorylation. In fibroblasts RAG-2 was phosphorylated predominantly at two serine residues, one of which affected RAG-2 activity in vivo. The threonine at residue 490 was phosphorylated by p34^cdc2 kinase in vitro; phosphorylation at this site in vivo was associated with rapid degradation of RAG-2. Instability was transferred to chimeric proteins by a 90-residue portion of RAG-2. Mutation of the p34^cdc2 phosphorylation site of the tumor suppressor protein p53 conferred a similar phenotype, suggesting that this association between phosphorylation and degradation is a general mechanism.