The products of the HLA-D region and their correlation to psoriasis vulgaris was studied using the primed lymphocyte typing (PLT) assay. In families with one healthy and one psoriatic parent and one psoriatic child primary MLC (mixed lymphocyte culture) stimulation was carried out between responder cells from the healthy parent and stimulator cells from the psoriatic child. In this way 21 PLT reagents directed against putative psoriasis-associated lymphocyte activating determinants were produced. Three reagents that gave clear bimodal stimulation patterns against lymphocytes of 51 psoriasis patients were further tested against 78 controls. Specificity of these reagents was studied using homozygous typing cells (HTC's) as stimulators. Compiled data show that cells from DR7 positive psoriatic patients give higher restimulation of these PLT reagents than do cells from healthy DR7 positive controls. In addition, a higher frequency of psoriasis patients compared to controls gave significant restimulation. Therefore we concluded that these PLT reagents recognized at least partly different DR7 associated determinants in the psoriasis patients and in controls. The reason is either that psoriasis patients carry a different lymphocyte activating DR associated specificity or, alternatively, that restimulation was caused by products of a distinct psoriasis associated locus in linkage disequilibrium with D/DR7 or of a determinant recognized together with D/DR7 as a restriction element. These data further support the notion that psoriasis is a disease with primary HLA associations to both class I and class II MHC genes.