We have analyzed the biosynthesis of casein kinase II. In exponentially growing tissue culture cells, the beta subunit was synthesized in excess of the catalytic subunit (alpha). A substantial fraction of newly synthesized beta was degraded within the first hour. The remaining fraction of beta was incorporated into holoenzyme. In contrast, little degradation of newly synthesized alpha subunit was observed and most was quickly and efficiently incorporated into holoenzyme. The assembly of beta with alpha was paralleled by an increase in apparent molecular mass of beta due to phosphorylation. The subcellular distribution of newly synthesized [35S]Met-labelled casein kinase II and of enzyme labelled and chased in the presence of excess unlabelled methionine was very similar and compatible with a nuclear localization. The degradation of the excess beta subunit occurred through a non-lysosomal proteolytic system with a very low ATP requirement.