We have characterized two monoclonal antibodies directed against rabbit thymocyte antigens. Using internally labelled MD3 cells (a transformed T-cell line), ART-F immunoprecipitated a membrane glycoprotein of 95,000 mol. wt, while ART-A immunoprecipitated two major glycoproteins, one of 95,000 mol. wt, the other of 105,000 mol. wt as analysed on reduced SDS-PAGE. ART-A recognizes 650,000 sites both on rabbit thymocytes and MD3 cells with an association constant of 1.5 X 10(5)/M. ART-F shows a biphasic binding curve with a high affinity constant of 3.3 and 0.6 X 10(8)/M and a low affinity constant of 2.2 and 6.9 X 10(5)/M for thymocytes and MD3 cells respectively. The numbers of high affinity and low affinity antigens are 200,000 for the former and 800,000 for the latter. On the basis of the kinetic data, the difference between low and high affinity is attributed to the difference in association-rate constants. The affinity constants calculated from the reaction-rate constants are in good agreement with those obtained from equilibrium measurements. While ART-F prevents the binding of ART-A, ART-A does not inhibit the affinity binding of ART-F. The analysis of the equilibrium, inhibition and kinetic data allow us to present a model for the structural relation of the epitopes recognized on the T-cells by the two monoclonal antibodies.