Serum samples from 95 recipients, transplanted with kidneys from live related donors, were tested for the presence of panel reactive antibodies (PRA) in pre- and post-transplant serum samples by the extended microdroplet lymphocytotoxicity test. The immunoglobulin class of antibodies was tested by treatment of serum with dithiothreitol. A significant correlation was found between the high PRA found in the 75 pretransplant sera tested and the subsequent rejection episodes. In addition, the level of pretransplant PRA activity was associated with graft survival in that patients with low PRA had significantly superior graft survival than those with high PRA. Furthermore, the present data show that patients with historical high PRA, but current low PRA, had graft survival similar to that in recipients who had moderate PRA in their current sera. High PRA patients had more often a positive crossmatch than patients with low PRA. The PRA level was also associated with prolonged waiting period. Immunoglobulin class of antibodies was related to graft acceptance in that the presence of IgM antibodies was not detrimental to transplantation. The results in the present study suggest that PRA of < 10% is negligible, while more attention should be paid to patients with PRA > 10%.