There are a number of polymorphic genes in mice which control the expression of hemolytic complement levels. The levels of hemolytic complement vary greatly within a non-inbred strain, CF-1, and there is a characteristic level of hemolytic complement activity for each inbred strain. Studies of progeny of crosses between selected inbred strains suggest that the levels of hemolytic complement are controlled by relatively simple genetic mechanisms. The significance of recognizing these genetic variants in understanding the functions of the complement system is discussed.