The number of histocompatibility loci at which the inbred mouse strains, C57BL/6By and BALB/cBy, differ was estimated in three independent experiments. The first experiment was based on survival of parental inbred strain grafts on F2 hosts and provided an estimate of at least 17 loci; the second was based on survival of grafts exchanged between sibs of partially inbred lines and provided an estimate of 28 loci; and the third was based on survival of skin from fifth generation backcross mice grafted on parental strain hosts and provided an estimate of 29 loci.