Abstract An air-driven ultracentrifuge has been used for the quantitative analysis of the interaction of concanavalin A with a number of serum glycoproteins. Concanavalin A significantly affected the sedimentation equilibrium behavior of 125I-labeled human α 1-acid glycoprotein, rat α 1-acid glycoprotein, and rat transferrin containing two N-acetyl neuraminic acid residues per molecule (Tf 2). The weight-average molecular weight of the labeled component increased and there was a corresponding decrease in the concentration of the labeled component at the meniscus. In contrast, concanavalin A did not significantly alter the sedimentation equilibrium behavior of the 125I-labeled rat transferrin containing three N-acetyl neuraminic acid residues per molecule. Sedimentation equilibrium results for interacting mixtures of concanavalin A and labeled glycoprotein and at various concentrations of the competitive inhibitor α-methyl mannoside were analyzed in terms of a model. Values of 5 × 10 4, 2 × 10 5, and 6 × 10 5 m −1 were obtained for the equilibrium constants for the interaction of concanavalin A with Tf 2 human α 1-acid glycoprotein, and rat α 1-acid glycoprotein, respectively.