Binding of Saccharomyces cerevisiae alpha-agglutinin to target a cells was assayed by agglutination inhibition and 125I-alpha-agglutinin binding. The assays showed characteristics of equilibrium binding, namely saturability, competability, and the establishment of a kinetic endpoint in the presence of free alpha-agglutinin and free receptor. The binding was heterogeneous, displaying strong binding (10(9) liters/mol) and a weaker interaction. There were about 2 X 10(4) strong binding sites per a cell. Denaturing gels displayed identical labeled species binding to the a cells in the weak and strong interactions. Furthermore, weakly bound material could subsequently bind tightly to fresh a cells, implying that the same species of alpha-agglutinin was bound in the two states.