A non-H-2 hemagglutinogen, rho, present in strain RFM mice is described. Genetic tests indicate that the factor segregates as a single antigenic unit that is independent from H-1 through H-4. Strain and tissue distributions indicate that the factor is not H-5 or H-6; strain distribution suggests that rho is not one of the described alleles of H-7 through H-11. Although an immune response was noted between strains RFM, rho positive, and A.CA, rho negative, in skin graft and tumor tests, serological analysis revealed that a locus other than rho was responsible for the moderately strong histocompatibility response. The antibody formed against rho is a leukoagglutinin and cytotoxin as well as a hemagglutinin.