Genetic control of bone mass in the spontaneously osteoporotic mouse (SAM-P/6) was examined by classical cross-mating experiments, in which SAM-P/2 with a high bone mass was used as a control. Femoral bone mass corrected by size (= bone density) was assayed photometrically. F1 hybrids exhibited intermediate values between those of the two parental strains, i.e. higher than SAM-P/6 and lower than SAM-P/2. F2 hybrids showed a more widely distributed pattern with a mean value quite similar to that of F1 hybrids. The values for backcrosses were between those of F1 and the respective parental strains. They did not show segregations expected in cases of a single-gene control. Numerical analysis of data distribution in each generation suggested that strain-specific bone mass in these strains of mice is inherited as a polygenic characteristic and controlled by a relatively small number of genes.