SUMMARYThe human allogeneic serum RM3 recognizes a lymphocyte structure inherited with the HLA chromosome. Population studies show several positive associations with antigens of the HLA‐A and HLA‐B series. Negative associations have been found with antigens of the HLA‐C series. Moreover the distribution of the RM3 factor is in Hardy‐Weinberg equilibrium with that of the HLA‐C alleles.However, serological investigations (namely the ‘lysostrip’ technique) bring evidence that RM3 serum does not appear to contain antibodies directed against any of the known antigens of the HLA‐A, B and C series. The study of a segregant family, showing a crossing over between the HLA‐A and HLA‐B loci, shows that the RM3 factor segregates on the side of the HLA‐A locus. In conclusion, within the limits of the serological approach used, the present study presents evidence of a new serologically defined HLA locus, different from t