An immunohistological study of skin biopsy specimens from patients with early syphilis was undertaken before and after treatment (one day after intramuscular administration of 2.4 MIU benzathine penicillin and eight days later, after a total administration of 3.6 MIU. In chancres from seronegative patients treatment with 3.6 MIU usually resulted in fewer immunocompetent cells in the infiltrate. In lesions of secondary syphilis treatment with 2.4 MIU benzathine penicillin produced a significant decrease in immunocompetent cells. After treatment with 3.6 MIU there was no further decrease. It was worth noticing that even eight to nine days after the initial pretreatment biopsy, when 3.6 MIU had been administered, the overall lymphohistiocytic infiltrate was not substantially diminished. Significantly more suppressor (T8+) cells were found in lesions of primary syphilis than of secondary syphilis, and they showed remarkable exocytosis. Activated local T8+ cells may release immunosuppressive lymphokines.