For the past three years, we have routinely used the one‐hourin vivosurvival of51Cr‐labeled donor red blood cells to select units for transfusion to patients for whom crossmatch compatible blood was unavailable. This technique successfully evaluatedin vivocompatibility, thus avoiding acute hemolytic transfusion reactions in 38 problem patients. New or confirmatory data regarding the hemolytic potential of several well defined antibodies was also obtained.Antibodies which proved to be clinically insignificant included: anti‐IT(all IgG), anti‐Sda, anti‐Kir, Mil, Oca, anti‐Chido, anti‐Bg, 14 of 15 “nonspecific warm autoantibodies” (three of which were associated with the ingestion of alphamethyldopa), and four of five antibodies to high‐incidence antigens. Clinically significant antibodies included: anti‐Yta, anti‐Jkb, the antibody in PCH (with “P” specificity), one intensely hemolytic “nonspecific warm autoagglutinin,” and one of five incompletely characterized antibodies to high‐incidence antigens. An acceptablein vivocompatibility test in every instance was associated with an appropriate rise in hematocrit and no clinical symptoms