A blood group system in swine is described involving 4 factors, Ea, Eb, Ec and Ed. Four combinations of these 4 factors have been observed. The genetics of the system are consistent with the existence of 4 alleles. When the gene for factor Ea is present on a chromosome as 1 member of the allelic pair, the other 3 genetic factors are missing. The corresponding allele is identified, therefore, as Ea - - -, the dashes representing the absence of the genetic factors of the other 3 known antigens of the system. A chromosome not possessing the genetic factor for antigen Ea appears always to carry the genetic factor for Ed and at least 1 of the 2 remaining genetic factors and may carry all 3:e.g.-EbEcEd, -Eb-Ed, or - - EcEd, the dash again representing a missing genetic factor. An animal may possess a combination of any 2 of these 4 groups of genetic factors. That these groups of factors behave as alleles is demonstrated by the fact that when a chromosome possesses any of the groups described above, these factors appear to be inherited as a block or complex similar to those in some blood group systems of cattle. The system exhibits no dominance; each character present can be detected in the corresponding phenotype, whether in the homozygous or heterozygous state. The inheritance of this blood group system is illustrated by the results of tests of the red cells of the sires and dams of 8 matings and their offspring with antisera specific for the 4 factors. In each litter the phenotypes of the young agree with the mode of inheritance described.