To observe effectiveness of the various crossing systems on the productivity in mice, the weights of products and efficiency of production which were composed of weights of all of the progeny at weaning and weight gain of both parents over the fifth parity were analyzed. Heterosis percentages computed on the weights of products ranged from 1.9% to 9.9% in the single crosses and from 25.4% to 33.7% in the back crosses, while those on the efficiency of production were in between 2.4-9.5%, 20.3-27.6% and 13.0-24.1%, respectively for the single, back and 3 way crosses. To find out the effectiveness of crossing between lines of mice as a model for livestock crossbreeding, calculation of production efficiency was attempted over the 5th parity.