The authors discuss the problem of resistance of chromomycosis to 5-fluorocytosine (5-FC), observed in a previous study in 7 patients initially treated with the drug and subsequently treated with 5-FC associated with other medicaments. All the associations, including that with amphotericin B, failed to overcome resistance. Experiments in mice and "in vitro" show a synergism between 5-FC and amphotericin B against "C. albicans", "C. neoformans" and "Fonsecaeae pedrosoi" already confirmed in human subjects infected with the above mentioned fungi. Resistance to 5-FC seems to be irreversible, as shown in previous experiments. The mechanism of action of the drugs on the fungi is discussed, as well as the fact that amphothericin B prevents the development of resistance to 5-FC. Results obtained with the use of the above association in 5 cases of chromomycosis are presented; all the patients had old and well established lesions, and in three cases these lesions were large and disseminated, conditions probably associated with the development of resistance to 5-FC used alone. The drugs were given in small doses, inferior to those usually recommended. The good therapeutic results seem to confirm the synergism observed in laboratory trials. Four cases were discharged as cured and one abandoned treatment for personal reasons. The association seems to act more rapidly than 5-FC alone. Tolerance was excellent in all cases.