Annexin II (p36) and p11, which belong to two different families of calcium-binding proteins, are able to form a heterotetrameric protein complex (p36)2(p11)2 called calpactin I. As these proteins were detectable only in the presence of each other in a variety of cell lines, we studied the mechanisms of regulation of cellular levels of annexin II and p11. In cells expressing p11 messenger RNA, p11 protein is undetectable unless annexin II is also expressed. As an example, the hepatoblastoma HepG2 cell line displays no detectable annexin II nor p11 protein, although it expresses p11 mRNA. The overexpression of annexin II by gene transfer into HepG2 cells leads to the up-regulation of the cellular levels of p11 by a post-translational mechanism. In the presence of annexin II, there is no major change in the p11 transcript levels, but the half-life of the p11 protein is increased more than 6-fold. Thus, the degree of expression of annexin II, which varies according to different states of cellular differentiation and transformation, is an essential factor in the regulation of cellular levels of p11.