Immunocytochemical methods to determine the ABO blood group of each blood of mixed bloodstains have been developed. Mixed bloodstains were made on surgical blades and a cedar board. The blades were dipped into blood and then dipped into blood of a different group at intervals of 30, 20, 15, 10 and 5 s. Two drops of blood were dropped on a cedar board and then two drops of blood of a different group were dropped there at the same intervals. The bloodstains were dried for a week. The blood samples were removed from the blades or the cedar board and processed according with a routine histological method. Three serial thin sections were obtained. After deparaffinization, the sections were treated in papain solution for 2 h at 36°C, to unmask antigenic sites on red cell membranes. The labeled streptavidin-biotin (LSAB) and peroxidase-anti-peroxidase (PAP) methods were used to detect A and B antigens, and an indirect immunocytochemical method for H antigen. These immunocytochemical methods showed specific immunologic reactions and allowed determination of the blood group of each blood of mixed bloodstains. Further, these methods indicated a possibility to determine who was stabbed first, in cases where two or more victims were stabbed with a single knife.