The titres of treponema-specific and antilipoidal 19S(IgG) antibodies were determined in rabbits infected intratesticularly with Treponema pallidum. One group of rabbits was treated with penicillin the other served as control. Using different serological tests it was shown that 19S(IgM) antibodies were still detectable eight months after infection at about the same titres in both groups. In contrast, 19S(IgM) antibody titres in patients with syphilis became undetectable within three to six months after penicillin treatment. It is suggested therefore that the rabbit is not a reliable model for studying the effect of penicillin in human T pallidum infections.