The presence of three trypsinogens (Try-Ill, Try-L and Try-II) in the mouse is demonstrated by DEAE-cellulose column chromatography. Two genetic variants of Try-I are detected, because the activity of Try-I is different between the Mol-A strain and seven other strains. The Prt-3 locus controls the activity of Try-L The Prt-3 a gene exists in CFO, BS, KR, BALB/cJ, C57BL/6J, CBA/J, and 129/Sv-S1-CP strains, whereas the Prt-3 b gene is present only in the Mol-A strain. Each Try-I from the CFO or Mol-A strain was purified. The fact that Try-I activity of the Mol-A strain is much higher than those of other strains is because of a difference in the specific activity; the ratio of the kc.t (sec -1) value with Tos-Arg-OMe to that with Bz-Arg-OEt is different between the variants from the CFO and those from the Mol-A strains, being much higher in the Mol-A strain. Also, chicken ovomucoid inhibited Try-I activity of the CFO strain at a molar ratio of one ovomucoid to two trypsins; Try-I activity of the MoI-A strain was only 50% inhibited even with an excess of ovomucoid. There was no difference between genetic variants of Try-I in molecular weight, K m values with Bz-Arg-OEt or Tos-Arg-OMe, pH optimum profile, or inhibition by soybean trypsin inhibitor.