The dikaryotic phase ofU. hordei, generally considered to be obligately parasitic, was forced to grow in culture by mixing auxotrophs of opposite mating types together in pairs on minimal medium. This in vitro system was used to carry out complementation tests with a group of 58 different arginine-requiring mutants, which were thus classified into 10 different complementation groups. More efficient methods for mating type determination and for screening of particular mutants from certain crosses were described.