Significant differences in relative weight of the levator ani muscle (LAW) between inbred and congenic mouse strains, differing genetically only by the major histocompatibility H-2 complex were found. It is assumed that a genetic factor (Hom-1), identical or closely associated wi th the H-2 complex, is one of the genes which influence LAW. These experiments suggest that for assay methods for myotropic activity of androgens groups of animals may be used with a homogenous genotype.