A search for practically valuable mutant genes conferring resistance to barley powdery mildew applied a screening system where four commercial varieties of barley without known genes for resistance were tested to the successful mildew race D1. Ionizing radiations and chemicals were used as mutagens. Among the 71 resistant lines isolated, 28 were recessive and belonging to locus ml-o, and 43 were dominant. In tests to a panel of 30 different races of barley powdery mildew, 27 among the dominant lines showed resistance spectra similar or identical to those characterizing 7 previously known genes. The remaining 16 lines formed 12 groups displaying new spectra of resistance to the panel of mildew races. Taking the frequency of recessive mutations in locus ml-o as a standard, ionizing radiations were significantly more efficient mutagens for producing dominant resistance to mildew. In spite of some possible sources of contaminant dominant resistance genes, the evidence points at a vast majority of the isolated lines representing genuine dominant mutations. Being produced in a single mutational step, such dominant genes for resistance offer evidence of having a regulatory function, regulating aspects of the host metabolism of importance for or interfering with the pathogen development.