A total of 15 strains of mice (11 inbred lines, 4 outbred stocks) of the institute for animal breeding of the veterinary faculty of the university of Munich (FRG) were scored for 23 biochemical loci each. The allelic distribution and frequencies were recorded as a basis for a so far not established genetic monitoring system. The inbred strains revealed the expected homozygosity with one exception (strain UMDH/+; locus Es1). The outbred stocks showed a remarkable degree of variable loci (17.4-36.3%) with a mean heterozygosity between 0.05 and 0.12. The markers in use allow differentiation between all strains (except for one pair of sublines) and stocks. 14 of the loci show allelic differences among the strains and thus can be used for a genetic monitoring system.