The behavior of monoclonal antibodies at high concentrations is important in downstream processing, drug formulation, and drug delivery. The objective of this study was to evaluate the osmotic pressure of a highly purified monoclonal antibody at concentrations up to 250 g/L over a range of pH and ionic strength, and in the presence of specific excipients, using membrane osmometry. Independent measurements of the second virial coefficient were obtained using self-interaction chromatography, and the net protein charge was evaluated using electrophoretic light scattering. The osmotic pressure at pH 5 and low ionic strength was >50 kPa for antibody concentrations above 200 g/L. The second virial coefficients determined from the oncotic pressure (after subtracting the Donnan contribution) were in good qualitative agreement with those determined by self-interaction chromatography. The second virial coefficient decreased with increasing ionic strength and increasing pH due to the reduction in intermolecular electrostatic repulsion. The third virial coefficient was negative under all conditions, suggesting that multi-body interactions in this system are attractive. The virial coefficients were essentially unaffected by addition of sucrose or proline. These results have important implications for the analysis of protein–protein interactions in downstream processing at high protein concentrations. Biotechnol. Bioeng. 2014;111: 529–536. © 2013 Wiley Periodicals, Inc.