During the VIIth Histocompatibility Workshop, 44 sera were selected that defined B‐cell alloantigens showing correlation with the HLA—D antigenic determinants. The segregation of these antigens, now called DRw (HLA—D‐related), was studied in a family that contained one HLA‐A/B recombinant and one HLA–B/D recombinant sibling.It could be established that the DRw‐1 antigenic determinants segregated with the B‐D region in the A/B recombinant sibling and that the DRw‐2 antigen segregated with the D region of the B/D recombinant child. In the second family, DRw typing showed that one of the children was identical with an HLA–ABC non‐identical sibling. This suggested that a crossing‐over had occurred. This was confirmed by mutual non‐stimulation of these HLA‐ABC non‐identical children in the mixed lymphocyte culture. This crossing‐over involved the DRw‐3 alloantigen.These data confirm the assumption that serologically defined B‐cell alloantigens are coded for by genes located outside the HLA‐A/B region on the B‐D side of the chromosome. Moreover, these data suggest that, within families, serological B‐cell genotype identity can predict a mutual unresponsiveness in the lymphocyte culture.However, findings in these two families indicate that discrepancies nevertheless exist between B‐cell serology and mixed lymphocyte reactions and that mutual stimulation can occur despite apparent identity of serological B‐cell phenotype and that B‐cell phenotype identity does not necessarily predict mutual u