One hundred and three recipients of kidney transplants from living related donors were analyzed for the influence of HLA-D matching on graft survival. Sixty-two donors were parents and 41 brothers or sisters. Twenty-two of the siblings were HLA-A, B identical. The degree of HLA-D compatibility was measured in a mixed lymphocyte culture reaction. Whereas nonresponsiveness was found with HLA-A, B identical sibling donors, the haploidentical donors were found to initiate varying degrees of stimulation. The stimulation was, calculated as the relative response (RR) to the donor in relation to an unrelated individual or to lymphocytes pooled from a number of individuals. It was found that RR values above 55% predicted a graft survival that was significantly inferior to the group with values less than 20% (p less than 0.01) and also to the group with RR values between 20% and 55% (p less than 0.05).