The purpose of this study is to determine if azithromycin, a drug approved for treatment of other infections, is as effective for syphilis (a sexually transmitted disease) as the standard treatment. Approximately 600 healthy adults, who are HIV-negative, ages 18 to 55 years of age, with primary, secondary or early latent syphilis, will participate in this research study. Volunteers will be enrolled in 5 U.S. cities and in Madagascar. Participants will be chosen randomly (by chance) to receive 1 of 2 study drugs: benzathine penicillin given (2 shots in the buttocks) or 4 tablets of azithromycin. Subjects who report a history of a penicillin allergy will be given either 2.0 g of oral azithromycin or 100 mg doxycycline taken orally, twice a day for 14 days. Over 2 years, 10 visits will be required. Procedures will include blood samples, physical exams, and swabs of sores.