An HLA--D homozygous typing cell -- MS -- was found to induce typing responses with a phenotype frequency of 7% tested in a panel of 202 random individuals. All but two of these were positive for HLA--Dw7, which implies that the MS cell defines a split of Dw7. The new subgroups, MS, is associated with B12. Three individuals positive for MS but lacking Dw7 were found to be B13 positive. The relationship of the MS specificity to HLA--Dw11 has yet to be defined.