The mixed lymphocyte endothelial cell culture was studied by the primed lymphocyte typing (PLT) technique. By comparing the HLA-D/DR specificity of the secondary response when using either peripheral blood mononuclear cells (PBM) or endothelial cells from umbilical cords for priming or restimulation of lymphocytes, it was found that PBM from newborns would induce a clear-cut specificity for HLA-DIDR when used for priming as well as for restimulation. N o HLA-DIDR specificity was seen, however, when endothelial cells were used for restimulation of lymphocytes primed to HLA-DIDR on PBM. On the other hand, lymphocytes primed to endothelial cells showed significant, albeit not very strong specificity for HLA-D/DR when restimulated with PBM. Our experiments suggest that HLA-D region products are present on endothelial cells, and thus confirm and extend serological studies using anti DR antisera.