Combined immunodeficiency disease (CID) is a genetic disorder of T and B lymphocyte production which results in a nonfunctional immune system. It is inherited as an autosomal recessive trait and was reported in humans and in horses of the Arabian breed. Arabian horses known to have the CID gene and horses of unknown carrier status were tested using a microlymphocytotoxicity technique. Computer .chi.2 analysis distinguished 6 serologically defined specificities. The study of unrelated horses and a limited number of families showed that the specificities behave as codominant alleles segregating at 1 locus. No differences in antigen frequency was detected between the CID carriers and the random horse population.