The sedimentation behavior of canine apolipoprotein (apo) A-I in 0.02 M EDTA, pH 8.6, was studied as a function of protein concentration by the techniques of sedimentation velocity and sedimentation equilibrium in the analytical ultracentrifuge. At concentrations of less than 1 g/liter, apo-A-I exhibited a monomodal sedimentation pattern, with apparent sedimentation coefficients which varied from 2.3 to 3.5 S with increasing protein concentrations. Above 1.5 g/liter, apo-A-I had two well resolved peaks with s20,w values of 4.15 S and 5.75 S. The proportion of the 5.75 S component increased with increasing apo-A-I concentrations, with a concomitant decrease of the 4.15 S component. By sedimentation equilibrium ultracentrifugation with both the conventional and meniscus-depletion methods, the apparent weight-average molecular weight of apo-A-I was found to be concentration-dependent. At a protein concentration of 5.25 g/liter, an apparent weight average molecular weight of 138,000 was determined, indicating that molecular species larger than a tetramer (monomer molecular weight = 28,000) were present in solution. When analyzed in terms of a reversible self-associating system, the experimental data could best be described according to a monomer-dimer-tetramer-octamer model, as previously reported from human apo-A-I (Vitello, L. B., and Scanu, A. M. (1975) J. Biol. Chem. 251, 1131-1136). The equilibrium constants were: K2 = 4.5 liters/g, K4 = 470 liters3/g3, and K8 = 41,600 liters7/g7, respectively.