The AutoAnalyzer method of red blood cell antibody detection can be adapted to identify the serological specificity of these same antibodies. In the present study, 1,152 AA-positive sera obtained during AA screening were investigated. In 217 (19%) of these sera blood group antibodies were identified by manual techniques. Since the AA techniques are generally more sensitive than manual methods an attempt was made to identify antibodies by the AA methods in 823 sera which were negative manually. The low-ionic strength-Polybrene (LISP) method was found to be suitable for antibody identification because of its sensitivity in the detection of most red cell antibodies and the small technical deviations of OD from the base-line. The bromeline-methyl cellulose (BMC) technique is less sensitive and often more difficult to interpret in antibody identification. In some cases, however, this method was the only one that gave positive reactions. In 214 instances antibodies were identified by the AA techniques, about half of which were anti-D’s, the others being RBC antibodies of more or less rare varieties. The relative frequency of these rare antibodies was higher in the group of manual-negative sera, 47 versus 21%, normally found. Positive identification but no clear-cut blood type antibody was found in 155 cases (19%). Indications were obtained that some of the reactions were due to HL-A antibodies. Antibodies to HL-A7 and HL-A5 were found to react with red cells. In addition the AA techniques seemed to be more sensitive to detect RBC autoantibodies than standard manual methods. However, the usefulness of the AA techniques for routine antibody identification is diminished by the relative slowness of the machine.