Histidine decarboxylase was purified from fetal rat liver about 3,000-fold by the method described previously (Watanabe, T., Nakamura, H., Leu, Y.L., Yamatodani, A., and Wada, H. (1979) Biochem. Pharmacol. 28, 1149-1155) except that DEAE-cellulose column chromatography, Sephacryl S-300 gel filtration, and preparative polyacrylamide gel electrophoresis were added. The enzyme had a molecular weight of 110,000 in the native state and gave a single band of protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis in a position corresponding to a molecular weight of 54,000, indicating that it was a dimer. Its isoelectric point was pH 5.1. Antibody raised in rabbits against the enzyme gave single fused precipitation lines with the enzymes from fetal rat liver and adult rat brain and stomach in Ouchterlony's double diffusion test, and inhibited these three enzymes similarly and strongly. Using this antibody, histidine decarboxylase-like immunoreactive structures were located in fetal liver and peritoneal mast cells, stomach and brain of rats.