ABSTRACT:There is a yearly outbreak of endemic leg ulcers in Thailand Corynebacterium pyogenes, a pathogen to animals, is frequently isolated from the fresh lesions in addition to common pyogenic bacteria such as beta‐hemolytic streptococci mostly group A, and Staphylococcus aureus and Staphylococcus epidermidis. Although circulating titers of IgG and IgM anti‐C. pyogenes antibodies were found to be very low in studied cases, intradermal skin test with C. pyogenes vaccine containing nonviable 105microorganisms induced an erythematous reaction with maximum intensity at 6 hours; direct immunofluorescence showed granular deposition of C3 along the dermoepidermal junction at this time. Unexpectedly, the sites of such a skin test that subsequently underwent biopsy presented tender erythematous induration 1 week later and developed granulomatous ulceralive lesions after 10 days in 11 of 15 patients.Radioimmunoassay for C5a and C5a of arginine demonstrated that C. pyogenes activaited complement even in the absence of specific antibodies. These findings suggest the importance of skin injury followed by infection with C. pyogenes in the development of endemic leg ulcers supporting our hypothesis that the infection of accidentally traumatized skin by C. pyogenes, which is carried by Oriental‐eye files, induces the granulomatous lesions of endemic leg u