Six syphilitic patients were treated with Penicillin in low dosis at the Evandro Chagas Hospital, of the Oswaldo Cruz Insitute. Protosyphilomas were found to be bacterioscopically-negative in between three and six days, and cicatrized completely between 10 and 13 days, under applicatins of 1.200 mucous patches, showed negative bacterioscopical results within between 10 and 20 days, under the applications fo 1.200 Oxford units per 24 hours. Doses of 4.800 units, during the same period of time, however, found insufficient to control other secundary luetic manifestations (roseolar rash). In three cases under sorological control it was found that after a clinical cure had been effected, the Wassermann reaction showed considerable escillation, becoming first negative and then pesitive again.