Twelve patients with syphilis were treated weekly with injections of 1.44 g (2.4 X 10(6) IU) of benzathine penicillin G for up to 3 weeks. Almost daily, serum penicillin concentrations were measured by a sensitive microbiological agar cup method. An individual and interindividual variation was found. Concentrations below the recommended 0.018 micrograms/ml were found 7 days after the first or second injection in 5 samples. Shorter intervals between injections are recommended.