Splenectomized patients with hemoglobin (Hb) Koln have rigid RBCs with membrane polypeptide aggregates that are not dissociable with disulfide- reducing agents. Malondialdehyde (MDA) action on normal RBCs produced rigid RBCs with similar nondissociable aggregates. To test the hypothesis that Hb Koln RBC aggregates contained unsaturated MDA-type bonds, we reduced normal control RBC membranes, Hb Koln RBC membranes, and MDA-reacted membranes with [3H]NaBH4. Hb Koln RBC membranes and MDA- reacted membranes both had significantly more 3H incorporation than control membranes. Furthermore, 3H incorporation in both Hb Koln and MDA-treated membranes was located in the membrane polypeptide aggregates, presumably saturating the crosslinking bonds. After reaction of RBCs with [14C]MDA, the MDA label was similarly concentrated in the membrane polypeptide aggregates. Normal RBC membranes incubated with MDA were analyzed with and without reduction by NaBH4 prior to amino acid determination by high-performance liquid chromatography (HPLC). Reduction with NaBH4 after MDA treatment decreased the lysyl residues by 33% and the serine by 7% and increased by 10% the methionyl residues, but did not affect 12 other amino acids. Similar changes could be detected in NaBH4-reduced Hb Koln aggregates in methionine and serine content. MDA may also alter protein configuration, as evidenced by an increase in the protease susceptibility of membrane proteins from MDA-treated and Hb Koln RBCs. We conclude that Hb Koln RBC membranes, like MDA-treated membranes, have similar high molecular weight aggregates conferring decreased membrane deformability, [3H]NaBH4-reducible unsaturated bonds, changes in amino acid composition upon reduction, and protease-sensitive configurational changes.