A specific antigen was found in each of four races of Melampsora lini and four rust-differentiating varieties of Linum usitatissimum. Avirulence and virulence were related to resistance and susceptibility through the specific rust antigens. A race was virulent to varieties containing its specific rust antigen as a minor constituent, and avirulent to varieties lacking that antigen. No such correlation was found with flax antigens.