A simulation is developed that qualitatively describes the small-zone-gel-filtration behaviour of a reversibly associating protein. The results reflect the dependence of the apparent molecular weight of a reversibly associating protein on the equilibrium constant (KD) and initial concentration of the protein as well as the column length. The behaviour of a protein on an individual column is characterized and thus a means is provided for estimation of KD. The procedure is extended to describe the behaviour of a mixture of two proteins capable of heterologous as well as homologous association. This computer simulation has been applied in association studies of immunoglobulin light chains [Stevens, Westholm, Solomon & Schiffer (1980) Proc. Natl. Acad. Sci. 77, 1144--1148]. The KD value determined for the Bence--Jones protein Au (10(5) M-1) is close to the value (6.6 X 10(4) M-1) determined by other methods [Maeda, Steffen & Engel (1978) Biophys. Chem. 9, 57-64].

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