There is a pronounced difference between the action of antibiotics and nitrofurans on Euglena gracilis. Those antibiotics that induce hereditary loss of chloroplasts do so only when they affect dividing cells. On the other hand, nitrofurans induce a mass mutation in both dividing and nondividing cells (under conditions of continuous illumination of cultures). It was found that a breakdown product, 5-nitro-2-furaldehyde, is liberated from furadantin and furoxone. This intermediate is responsible for the observed specific mutagenicity of 5-nitrofuran drugs. The mutagenic action of 5-nitro-2-furaldehyde is very similar to that of nitrosoguanidine. Both compounds induce bleached mutants of E. gracilis when acting on growing or resting cells, regardless of the dark or light conditions. Similarly, both compounds induce reverse mutations in auxotrophic strains of Mycobacterium phlei.