Purified suspensions of highly viable parenchymal, endothelial and Kupffer cells were prepared from the livers of young (3 months of age) and old (33-34 months of age) female rats. In the cell suspensions, the activities of the multiple forms of acid phosphatase were determined with the substrates 4-methylumbelliferyl phosphate, 1-naphthyl phosphate and p-nitrophenyl phosphate. Most specific enzyme activities increased during aging, but the preference to hydrolyse any of the three substrates did not change. The small age-related changes in effects of the inhibitors fluoride and alloxan were not uniform for the the three liver cell types. The pattern for multiple forms of acid phosphatase obtained after isoelectric focusing did not change with age for Kupffer cells, while one of the two major forms of the enzyme almost disappeared in endothelial cells from old rats. With age, a very pronounced heterogeneous pattern was observed for the enzyme forms in the long-lived parenchymal cells. The increase in heterogeneity in multiple forms of acid phosphatase in aging parenchymal cells may be the result of post-translational modifications.