We report on flow cytometric IgG subclass determinations of red cell antibodies using polyclonal FITC-labeled antibodies. The limit of detection of this method was 1 ng anti-D per 1x10^7 red cells. The inter- and intra-assay coefficients of variance were 8.2 and 2.3%, respectively. In 8 newborns with a positive direct antiglobulin test (DAT) in the gel centrifugation test (GCT), due to ABO antibodies, IgGl was detected in all and IgG2 additionally in 4 of these cases. In 5 severe cases of hemolytic disease of the newborn (HDN) due to anti-D, large amounts of IgGl were found, and in 3 of these 5, IgG3 in combination with IgGl. In 8 mild or moderate HDN cases (4 anti-D, 2 anti-E, 1 anti-Fy^a, 1 anti-Jk^a), phototherapy sufficed, and IgGl was the only antibody. In 7 adult patients with malignant lymphoma and a positive DAT (GCT), only small amounts of IgG1 red cell autoantibodies could be demonstrated by flow cytometry. In 5 further patients with malignant lymphoma, a positive DAT, and severe hemolytic anemia, large amounts of IgGl autoantibodies were found and IgG3 was also present in 3 of these cases. Flow-cytometric determination of IgG subclasses may be a useful tool in immunohematology, since subclass determinations were possible in all of these cases. This method is suited for clinical routine and offers the possibility of sufficient standardization.