Seven patients in various stages of syphilis were treated by oral administration of amoxicillin (6 g daily) and probenecid (2 g daily) for 15 days. The treponemicidal level of amoxicillin was studied by a Treponema pallidum immobilization assay and found to be 0.070 micrograms/ml, as compared with 0.003 micrograms of penicillin/ml. Taking into account the WHO-recommended level of 0.018 micrograms/ml of penicillin, the minimal level of amoxicillin was estimated as 0.42 micrograms/ml. This level was obtained in serum and cerebrospinal fluid (CSF) of all patients treated with the above mentioned oral combination. It is concluded that treponemicidal levels of amoxicillin can be obtained in CSF after oral administration. The amoxicillin regimen described may be a valuable alternative to single-dose parenteral penicillin in the treatment of syphilitic patients with suspected CNS involvement, provided that reasonable compliance can be obtained.