Conidia of an actidione-sensitive wildtype strain of Neurospora crassa were irradiated with UV-light. They were then plated into nutrient-agar, either with or without actidione. The latter plates were incubated for several hours, before nutrient agar containing actidone was layered onto the plates. Colonies formed in both sets of plates were isolated as actidione-resistent. They were studied further by genetic and biochemical means. Pre-incubation of the irradiated conidia before subjecting them to the action of actidione increased the mutant yield considerably, as compared to immediate plating with the drug. E.g. a 13 hours pre-incubation gave ca. 100 times more resistent colonies than were obtained without pre-incubation (Fig. 2). Their resistent phenotype was stable on vegetative propagation. 17 mutants were mapped by crossing them with suitable tester-strains. Of them, 14 were found to belong to linkage group I, the remaining to linkage group V. The mutants are, therefore, considered as characterizing resp. genes act-1 and -2 of Hsu (1963). Act-1 and -2 mutants were crossed with suitable auxotrophic strains to obtain auxotrophic, actidione-resistent isolates. These were combined on minimal medium with auxotrophic, actodione-sensitive strains of the same mating type. Conidia of the arising heterokaryotic mycelia were tested on minimal medium with and without actidione. In these tests resistence of act-1 and -2 mutants was found to be dominant over the sensitivity of the wildtype. However, an analysis of nuclear ratios in the conidial populations by differential plating does not exclude incomplete dominance of act-1. Incorporation of 14-C-leucine into protein of conidia of the wildtype was strongly inhibited by 1 γ actidione/ml. Resistence in two mutants, representing the two separate genes, was accompanied by a marked decrease of this inhibition. No significant differences in the amount of inhibition were found between the two mutants. It is suggested that cytoplasmic ribosomes may be the cellular components influenced by actidione. In the case of the mutant cells the actidione is no longer effective in this capacity, possibly because of changes in the ribosomal proteins.