Recombinant DNA derived human interferon .gamma. (IFN-.gamma.) from Escherichia coli was examined by equilibrium ultracentrifugation. Short-column equilibrium experiments at pH 6.9 in 0.1 M ammonium acetate buffer gave a z-average molecular weight of 33,500 .+-. 1400 at infinite dilution, corresponding to 1.98 .+-. 0.08 times the formula weight. Long- (2.6 mm) column experiments at pH 7.5 in 0.04 M imidazole buffer gave a molecular weight of 33,400 .+-. 500. Under the latter conditions IFN-.gamma. behaves somewhat nonideally, with the departure from ideally accounted for by an effective (Donnan) charge of about 6+. No association of this dimer to form tetramer or higher polymers was observed, with the association constant for formation of tetramer from dimer K24 found to be less than 34 L mol-1. Similarly, no dissociation to monomers was observable, with the dissociation constant to monomer K21 being less than 5 .times. 10-8 mol L-1. At pH 3.55 in 0.02 M buffer (acetate plus acetic acid), there was virtually complete dissociation of the dimer to monomer. Extreme nonideality was seen in this low ionic strength system, and the effective charge on the protein was estimated to be about 11+. The reduced molecular weight M(1 -.hivin..nu..rho.) of the monomer was found to be about 4.09 .+-. 0.20 kg mol-1; this corresponds to a molecular weight of 16,410 .+-. 820, with the Scatchard definition of components. A small amount of a polymer with a molecular weight of about 0.5 .times. 106 was detected under these conditions.