Phosphorylation of two species of erythrocyte pyruvate kinase (PK, EC 2.7.1.40), PK-R_1 and PK-R_2, was analyzed. The light fraction of younger red cells which has a low PK-R_2/PK-R1 ratio, and the dense fraction of older cells which has a high PK ratio were used as the enzyme source. On phosphorylation in the presence of cyclic AMP, PK of the dense fraction showed a larger amount of ^32P incorporation than that of the light fraction. Most of the radioactivity incorporated was recovered in the L' subunit of PK. The unphosphorylated PK of the dense fraction had a lower affinity for phosphorylated PK of the dense fraction still showed a lower affinity for PEP than that of the light fraction. There results indicate that PK-R_1 and PK-R_2 are controlled by phosphorylation with a decrease of the affinity for PEP. The postsynthetic transformation of PK-R_1 into PK-R_2 is considered as a maturation process or molecular aging of the enzyme which involves protein degradation in erythrocytes.