As plant breeders select alfalfa (Medicago saliva) genotypes for improved digestibility by ruminants, there may be an increased risk of yield losses due to plant disease. This is because increases in digestibility are often associated with a decrease in lignin content and lignin has been shown to play an important role in plant defense mechanisms against disease and pests. The method most often employed by public and private alfalfa-breeding programs to assess digestibility is acid detergent lignin (ADL) analysis. ADL concentration was determined for individual alfalfa plants from two different alfalfa populations. Plants representing a range of ADL concentrations within each population were arbitrarily selected, cloned, and used in experiments to quantify the relationship between leaf ADL concentration and components of resistance to Uromyces striatus, the causal agent of alfalfa rust. Three components of resistance were quantified: infection efficiency (pustules per cm2 leaf area), latent period (the time from inoculation to when 50% of the pustules were visible), and sporulation capacity (the number of urediniospores produced per pustule). Although analysis of variance found significant differences among clones for infection efficiency, latent period, and sporulation capacity, regression analysis revealed little or no relationship between ADL concentration and components of alfalfa rust resistance. F statistics for regression equations and t statistics for slope parameters generally were not statistically significant and when these statistics were significant, coefficients of determination (r2) values indicated that ADL concentration explained only 23% or less of the variation in resistance components.