An antiglobulin test, more sensitive than established direct microcytotoxicity tests, was developed to detect both complement fixing and non-complement fixing alloantibodies. The test has been used to detect antibodies in sera from selected organ allograft recipients. Of 56 sera tested by the antiglobulin method, 52 (93%) were reactive. In contrast, only 13 (23%) were reactive in an established direct microcytotoxicity test and only 28 (50%) were reactive in a modification of the direct test in which the incubation times were doubled. Sera obtained prior to transplantation from 3 patients who experienced either immediate or early rejection of their renal allografts contained antibodies detected by the antiglobulin test but not by direct microcytotoxicity tests against lymphocytes from a panel of unrelated, normal individuals. In addition, the antiglobulin test has been used to demonstrate that the humoral antibody response following transplantation of cardiac and renal allografts is heterogeneous with respect to formation of antibodies of the IgA, IgG and IgM classes.