The abstract of this paper was presented at the 14th Meeting of the International Association of Forensic Sciences, Tokyo in 1996. We report a bizarre criminal case of suspected serial homicide by injection of a muscle relaxant (succinylcholine). Five victims were found buried in a rural area. In two victims showing moderate decomposition (about three months after death), intense pulmonary oedema with pleural effusion was observed. Evidence of a puncture site was found in one of the victims. Succinylcholine could not be detected in the victims, but was identified in a syringe found near the corpses. The 40-mg ampule dose of succinylcholine administered intramuscularly to the victims, possibly causing prolonged apnea, was considered to be at least around the minimum lethal dose, although the combined effect of the sedation with hypnotics also used was not negligible.