Interactions between human intestinal spirochaetes (HIS) related to intestinal spirochaetosis and intestinal pathogenic anaerobic bacteria were investigated by searching for the presence of cooperative haemolysis among 39 strains of weakly beta-haemolytic human intestinal spirochaetes and Clostridium perfringens alpha-toxin producers on plates carrying six different sheep blood agar media. An area of intense cooperative haemolysis (about 3-10 mm) was observed between all tested spirochaetal strains and C. perfringens where the clostridial alpha-toxin diffused toward the colonies of the spirochaetes overlapping part of their growth zone. The cooperative haemolysis was a potentiation of the haemolysis due to the single cultivation of human intestinal spirochaetes and C. perfringens and was observed after anaerobic incubation for 24-48 hours when both bacteria at a concentration range of 10(8)-10(3) CFU/ml were streaked at a distance of 3-10 mm to each other. A cooperative haemolysis was also observed between C. perfringens and weakly beta-haemolytic spirochaetes related to porcine and avian intestinal spirochaetosis and the spirochaete causing swine dysentery. The present study indicated that the damage produced in vitro by the clostridial alpha-toxin was enhanced only on the red blood cells which were in proximity to the HIS colonies causing the complete lysis of the erythrocytes. It is hence possible that the potentiation of the damage to red blood cells observed in vitro mimics an in vivo damage on the membranes of enterocytes to which HIS are attached when intestinal spirochaetosis occurs and when cytolysins similar to the alpha-toxin are available in the intestine of the host.