SUMMARYThe genetic control of the histocompatibility antigens that induce strong alloreactions in Syrian hamsters was examined. Genetic studies revealed that the alloantigens involved in skin graft rejection, graft-versus-host reactions, and mixed lymphocyte reactions are under dominant single gene control and that these genetic loci are closely linked. These data suggest that this strong histocompatibility locus (i) may represent the major histocompatibility complex equivalent in this species, and this locus or group of loci has been called Hm-1. In addition, studies concerning the genetic control of the immune response to bovine serum albumin suggest that the high response is under dominant, single gene control; however, this gene is not linked to Hm-1.