The first results of treatment of yaws by Penicillin are here reported. In seven patients a complete disappearence of the external lesions was obtained between the 12th and the 44th day of treatment, and serological reactions were negative in the 60th day in all cases. The observations of the patients will be continued for six months, under immunologic controls each 8 days. The treatment has been performed, using a mean dosis of 200 Oxford units each 4 or 6 hours. The total amount of units "per" subject, has varied from 9.600 to 52.000. No toxic phenomena due to the drug have been observed, in spite of the frequency and the high number of injections.