SummaryA previously unrecognized canine red cell antigen, tentatively named O, was found after absorption analysis of an alloimmune antiserum and absorption of several immune or naturally occurring cross‐reacting heteroantibodies from man, cattle and swine. The serological results and genetic analysis of a limited number of complete dog families indicated that the new factor probably belongs to the Tr blood group system. No individual possessed both factors Tr and O, and a large proportion of animals was negative for both factors. The serological pattern for the new Tr system obtained was consistent with similar systems observed in sheep (A‐O), pig (R‐O) and man (Bombay) in which the gene for one factor was dominant over and masked the gene for the second factor in the system. The negative phenotype was accounted for by the actions of an epistatic