REVIOUSLY published data (GORER 1942; GORER, LYMAN and SNELL P 1948) have shown that alleles at the histocompatibility-2 locus in the mouse have a dual effect, determining a blood group antigen and also susceptibility or resistance to certain transplantable tumors. The blood group is niost easily demonstrated with sera from C57BL mice which have received one or more inoculations of A strain tumor 15091a. Such sera will agglutinate A strain red cells, indicating that the tumor and the red cells

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... ossess an antigen in common. In crosses between the A and the C57BL strains, the A erythrocyte antigen and susceptibility to A strain tumors are always found associated ; hence they probably are determined by the same genetic locus. Three alleles have been recognized : H-2, characteristic of the A strain ; H-Za, found in the dba's ; and h-2, heretofore regarded as characteristic of C57BL, CBA and P mice. The interpretation of H-Zd as a separate allele rests on three serological observations. Strain dba tumor P1534 will elicit antibodies in C57BL mice, but at a lower titer than those produced by 15091a. Strain dba red cells tested against anti-15091a serum give a slightly lower titer than do A red cells. Finally, this antiserum when absorbed with dba red cells still reacts in a dilution of 1 to 8 with A red cells. Tests carried out by methods adopted to show-linkage between histocompatibility genes and known marker genes (SNELZ 1948) showed that H-2 is closely linked with the locus for fused tail, Fu. In the experiments previously reported, though there were several animals that can have been either crossovers, or normal overlaps in the expression of fused, no unquestionable crossovers were obtained. METHODS