Soybean trypsin inhibitor(Ti), antigrowth factor, from 731 cultivars (G. max) and 142 wild (G. soya) soybeans were assayed by PAGE, Four alleles of Ti^*a, Ti^*b, Ti^*c, and ti are codominant which are located on a single locus. The frequencies (0.6881±0.0029, 0.5880±0.0078, respectively) of cultivated and wild soybean Ti^*c alleles were found to be higher than those (0.2339±0.0029, 0.0915±0.0078, respectively) of the Ti^*a alleles of 731 cultivated and 142 wild soybean lines. The Ti^*b allele (0.0786+0.0029) was found only in the cultivars. Soybean is a highly self-pollinated crop. However, 17.1% (N=125) heterozygotes trypsin inhibitor genotypes are observed in cultivars, and 66.2% (N=94) heterozygotes observed in wilds. Protein content of cultivars differed in six trypsin inhibitor phenotypes with a mean of 42.02±1.44% (N=203) in Ti c-ctypes, 42.30±1.51% (N=83) in Ti a-a types, 41.30±1.44% (N=6) in Ti b-b types, 43.10±1.24% (N=12) in Ti a-c types, 42.43±1.41% (N=4) in Ti a-b types, and 41.97±1.37% (N=26) in Ti b-c types. These data suggest that heterosis for protein content revealed in cultivars.