Skin biopsies were obtained from an individual previously found to be homozygous for the HLA[human histocompatibility antigen] Dw3 determinant. MLC [mixed lymphocyte culture] and MLSC[mixed lymphocyte skin culture] techniques were used. Either lymphocytes and skin cells from the same individual express the same HLA-D locus products or the in vitro lymphocyte activating determinants on skin cells are different from those on lymphocytes, but are determined by genes in a very high linkage disequilibrium.