The variation from spontaneous mutations for 6-week body weight in the mouse was estimated by selection from a cross of two inbred sublines, C57BL/6 and C57BL/10, separated about 50 years previously from the same inbred line. Selection was practised high and low for 12 generations from the F2, followed by one generation of relaxation. The lines diverged by approximately 1.7 g or 0.7 sd. The additive genetic variance was estimated in the F2 by restricted maximum likelihood and from the selection response, and from this variance the mutational heritability hM2 was estimated using the number of generations since divergence. Estimates of hM2 range from 0.08 to 0.10% depending on the method of analysis. These estimates are similar to those found for other species, but lower than other estimates for the mouse. It is concluded that substantial natural and, perhaps, artificial selection operated during the maintenance of the sublines.