1. The dephosphorylation of 3'-AMP, 3'-dAMP, 3'-CMP and 3'-dCMP was studied in the postmicrosomal supernatant of rat spleen and liver. In both organs 3'-AMP and 3'-dAMP were dephosphorylated at an appreciable rate, in both the presence and the absence of Mg(2+). The pH optimum for this dephosphorylation was in the range 4.5-5.0. 3'-CMP and 3'-dCMP were very slowly degraded, though the activity towards 3'-dCMP increased somewhat in the presence of Mg(2+). The optimum pH for this Mg(2+)-dependent dephosphorylation was 5.5-6.0. 2. The rate of dephosphorylation of 3'-AMP and 3'-dAMP per mg of protein was about 5 times as high in spleen as in liver. 3. The dephosphorylation of 3'-AMP could be ascribed to a single enzyme with pH optimum about 4.5. The activity towards 3'-dAMP could be resolved into one component coinciding with the 3'-dAMP-degrading enzyme, and one Mg(2+)-requiring component probably identical with the soluble deoxyinosine-activated nucleotidase. The dephosphorylation of 3'-dCMP seemed to be performed only by the latter enzyme. 4. The enzyme dephosphorylating 3'-AMP was purified 200-fold from the postmicrosomal supernatant and its physical and catalytic properties were compared with those of acid nucleotidase (EC 220.127.116.11) purified from rat liver lysosomes. The two enzymes were identical in all properties tested (substrate specificity, K(m), molecular weight, response to phosphatase inhibitors), but some of the data differed from earlier reports on the acid nucleotidase. 5. The subcellular localization of the acid nucleotidase, its relationship to the acid phosphatase(s) and its role in the breakdown of nucleic acid constituents are discussed.