This study was undertaken to ascertain the incidence, the associated clinical diseases, and the serologic characteristics and significance of positive direct antiglobulin tests (DAT) in a hospital population. Forty-one (0.9%) of 4,664 medical-surgical patients had positive DAT's on admission. Eight of these 41 people also had antibodies in their sera. Twelve patients were taking alpha-methyldopa and one cephalothin, seven had malignant tumors, nine had immunologic disease, and 12 had miscellaneous conditions. Elution of antibodies from the erythrocytes of 36 patients was attempted: 17 eluates failed to react with panel cells; anti-K antibody was found in the eluates from two patients who had recently received transfusions; five eluates showed no blood-group specificity; and 12 agglutinated all panel cells. Eleven of the last 12 eluates, after absorption with Rh-negative cells, were tested with Rh cells of various phenotypes, and weaker or negative results with Rhnull cells were obtained with seven absorbed eluates. Three of the 41 patients had mild to overt hemolysis.