Various patterns of mouse pancreatic proteinase activity bands were observed on agarose gel electrophoresis. Prt-1a and Prt-1b genes control the positive (PRT-1A) and negative (PRT-1B) expression of tryptic band V, respectively; Prt-2a and Prt-2b correspond to chymotryptic bands II (PRT-2A) and III (PRT-2B); Prt-3a and Prt-3b control the low (PRT-3A) and high (PRT-3B) tryptic activities of band IV; the Prt-1 and Prt-3 loci are closely linked on the same chromosome; Prt-6a and Prt-6b correspond to tryptic bands I (PRT-6A) and I' (PRT-6B). Twenty-four laboratory strains from the United States showed the phenotype PRT-1A, PRT-3A, and PRT-2A. Of laboratory strains established in Europe, 6 showed PRT-1A, PRT-3A, and PRT-2A, and 10 had PRT-1B, PRT-3A, and PRT-2A bands. Most wild mice around the world and their descendants showed the phenotype PRT-1B, PRT-3B, and PRT-2A. Only the phenotype of M. m. brevirostris was PRT-1A, PRT-3A, and PRT-2A, which was the same as most laboratory inbred strains. PRT-2B was observed mainly in Japanese (M.m. molossinus) and Korean (M.m. yamashinai) wild mice. PRT-6B was detected only in Mus spicilegus and Mus caroli, but all other mice including wild populations and laboratory strains showed PRT-6A. New biochemical phenotypes such as PRT-2C and PRT-3C were also found in this study.