The mode of action of 5-fluorocytosine (5FC) and 5-fluorouracil (5FU) in dematiaceous fungi was studied and compared with results of experiments in yeasts and Aspergillus species. In dematiaceous fungi 5FU is more potent than 5FC. The high activity of 5FU is related to a good and rapid uptake of this compound into the fungus cell. Both compounds exert fungistatic and fungicidal activity. A correlation exists between the amount of 5FU incorporated into RNA and its antifungal activity. The resistance frequency to 5FC varies from 2 x 10(-3) to 1 x 10(-7); resistance frequency to 5FU is generally lower. Addition of 5FC and 5FU to logarithmically multiplying cells inhibits increases of cell numbers and cell constituents after a delay period. The effects on the increase of protein and carbohydrate are more delayed than on the increase of DNA and RNA, indicating unbalanced growth. The concept of a dual biochemical mechanism, i.e. incorporation of 5FU into RNA and formation of 5-fluorodeoxy UMP leading to inhibition of DNA synthesis, previously proposed for the antifungal action of 5FC is also applicable to the action of 5FC and 5FU on the dematiaceous fungi.