The need for anti-complement (anti-C') activity in antiglobulin antisera (AHG) for the detection of clinically significant antibodies was evaluated during a three-year period. While performing routine compatibility testing using standard blood banking procedures, eight patients were found whose antibodies were detectable primarily or only by AHG containing anti-C' activity; monospecific anti-igG AHG gave weak or negative reactions. Seven of the antibodies were anti-jka or jkb. Two of the anti-jka antibodies were responsible for clinically unsuspected delayed hemolytic transfusion reactions. The anti-jkb antibody resulted in a shortened survival of incompatible 51Cr-labelled red blood cells. The incidence of such "complement-only" Kidd antibodies was 23 percent of all Kidd antibodies found. These data suggest that the omission of anti-C' in AHG in routine compatability testing could result in substantial risk of failure to detect clinically significant antibodies.