RECENT reports have suggested the occurrence of DNA in mitochondria of yeast1 and chick embryo2. These studies have added interest in view of the presence of DNA in the chloroplast fractions of algae and higher plants3, and its obvious implications for non-chromosomal inheritance4. The respiratory-deficient petite mutants of yeast possess altered mitochondria5, and they are easily induced by a variety of agents6. It is important to establish the chemical nature of the non-chromosomal determinants of this inheritance pattern, with DNA and RNA most probably involved, since they are the principal genetic materials. To approach the question of which nucleic acid might be the determinant for the respiratory characteristic, Moustacchi and Marcovich7 added uridine and thymidine along with the mutagenic base analogue 5-fluorouracil (5-FU). Their data showed that added thymidine was ineffective at all mutagen concentrations used, but that uridine significantly reduced the frequency of mutants induced at 5-FU concentrations of 0.1 and 0.2 µg/ml. There was no appreciable effect when uridine was added at 5-FU concentrations lower than 0.1 or higher than 0.2 µg/ml. When both nucleosides were added, the results paralleled those obtained with only thymidine added or with neither nucleoside present. They concluded that RNA was more likely than DNA to be the determinant involved, even though the uridine effect occurred only in a narrow portion of the concentration range of the mutagen. We have reinvestigated the problem, using the same experimental design, with the more commonly applied mutagen for petite induction, the acridine dye acriflavin. Unlike Moustacchi and Marcovich7, we found that both uridine and thymidine reduced the frequency of petite mutations induced by acriflavin. Our results, like theirs, also showed the nucleoside effect to be restricted to a narrow segment of the range of mutagen concentrations tested.