Human pancreas is the source of an alanine aminopeptidase (HPAA) that is unique to pancreas and is readily distinguishable from the liver, kidney, and duodenal alanine aminopeptidases. Each of these three aminopeptidases appears in small quantities in blood and therefore may consitute tissue/organ specific marker enzymes. In this study alanine aminopeptidase from pancreas has been purified. Pancreas alanine aminopeptidase was resolved upon purification with ion exchange chromatography into three isoenzymes. Gel filtration chromatography of these isoenzymes indicates that their molecular weights are near 235 000 daltons. The isoenzymes contain a firmly bound divalent cation that could be removed with EDTA only at temperatures above 40 degrees C (at which the enzymes were stable) and below 52 degrees C (at which thermal denaturation begins to take place). Treatment with EDTA at 4 degrees C yields fully active enzymes, which, however may be stimulated approximately 200% by the addition of Co2+ at 10(-4) mol/l. This cobalt stimulation is easily reversed by dialysis of the stimulated isoenzymes against deionized water. The pancreas alanine aminopeptidases were not inhibited by tosyl-phenylalanine chloromethylketone or by tosylleucine chloromethylketone, whereas phenylalanine chloromethylketone was inhibitory. The Km values for methionyl-, arginyl-, leucyl-, alanyl-, and isoleucyl-beta-naphthylamide for each isoenzyme are statistically identical and the average values are 0.36, 0.60, 0.69, 1.25, and 1.29 X 10(-4) mol/l, respectively. The kcat values are 2.50, 1.58, 0.82, 0.38, and 0.19 X 10(4) sec-1, respectively.