This study compared flow cytometric analysis with tube agglutination assays for the detection of red blood cell (RBC)‐associated complement and immunoglobulins (Igs). RBCs from 20 patients with reactive tube direct antiglobulin tests (DATs) were evaluated by flow cytometry with anti‐C3d, anti‐IgG, anti‐IgM and anti‐IgA. Serial samples were also tested from a patient at risk of passenger lymphocyte haemolysis. Results of flow cytometry and tube assays for anti‐IgG were as follows: 12 of 20 samples reactive in both; six of 20 nonreactive in both; two of 20 discordant with a reactive tube and a nonreactive flow cytometry assay. Anti‐C3d results showed nine of 20 reactive in both and 11 of 20 discordant with a nonreactive tube and a reactive flow cytometry assay. In the IgM flow cytometry assay, three samples were reactive with anti‐IgM. Samples from a group A woman who was transplanted with stem cells from a group B donor showed that on days 3 through 6 post‐transplant, the flow cytometry assays for anti‐IgG and/or anti‐C3d were reactive, whilst the tube assays were nonreactive. In conclusion, flow cytometric analysis is more sensitive than the tube assay for the detection of RBC‐associated C3d. Further studies are needed to determine the correlation of C3d levels with clinical sequelae.