With 37 formalin-fixed, paraffin embedded specimens from the lesions of 30 patients with primary, secondary or gastric syphilis, we performed avidin-biotin-peroxidase complex (ABC), indirect immunoperoxidase (IIP) and FTA-ABS complement techniques. Darkfield examination was done in 17 skin lesions. The immunoperoxidase technique, especially the ABC technique, revealed higher reactivity than the FTA-ABS complement technique and darkfield examination in detecting Treponema pallidum in tissues. Furthermore, the ABC technique produced less intense nonspecific background staining than the IIP technique. Histologically, most of the treponemes were located in the upper dermis, epidermis and vessel walls in the order named, and rarely in the lower dermis of the syphilitic skin lesions.