Weak red cell sensitization by hemolyzing antibodies may not be detected by the standard antiglobulin test. For the diagnosis of certain immune hemolytic anemias more sensitive methods are needed. Anti-IgG was purified as F(ab)2 by pepsin digestion and labelled with 125 I using the chloramine T method. 15 X 10(7) cells were washed and suspended in 0.1 ml of 6% albumin in saline. After incubation with 0.1 ml of 125 I-anti-IgG F(ab')2 for 30 minutes, the cells were washed and the uptake of radioactivity was counted using a gamma-counter. Standard cells were coated with a known concentration of an IgG anti-D preparation in the range of 50--1000 molecules per red cell. The sensitivity for the detection of red cell sensitization was at least 50 molecules per red cell. Because of varying binding ratios between the labelled anti-IgG reagent and different red cell antibodies the determination of the exact number of IgG molecules on the red cells is difficult. However, this very sensitive and reproducible method shows practical advantages in comparison with a complement fixation antibody consumption method with which corresponding results were obtained. Beside the evaluation of antiglobulin test negative autoimmune hemolytic anemias, this method may be applied for the determination of the specificity of weak red cell alloantibodies not detectable by standard blood bank procedures.