Good saprobic growth of primary and established cultures of Melampsora lini (Ehrenb.) Lév., race 3, was obtained on chemically defined media containing modified Knop's minerals, various trace elements, 5% sucrose, and aspartic acid. Cysteine, cystine, and glutathione had to be added to the base medium singly or in combination. Alanine and methionine, added to minerals and sucrose, supported only sporadic and slow growth of primary flax rust cultures; and established cultures started on media containing more complex amino acid mixtures did not survive on the alanine–methionine medium. Types and concentrations of essential amino acids determined the rate of mycelial growth.