Abstract An observation of sudden loss or gain of mouse virulence of the mouse-adapted type III poliomyelitis virus, Leon strain (designated as LH substrain) was described. The virus lost its mouse virulence after one or two passages in monkey testicular tissue culture. This variant, designated as LH-a, remained mouse nonvirulent after 15 consecutive passages in monkey testicular tissue culture followed by 15 passages in monkey kidney tissue culture. However, it was again made mouse virulent at its 18th tissue culture passage by a single passage in monkey testicular or kidney tissue grafted onto chorioallantoic membrane (CAM) of chick embryos.