Size-exclusion high-performance liquid chromatography with a TSK 5000 PW column was shown to be a fast and relatively inexpensive method for the size determination of lecithin-bile salt mixed micelles. Perturbation of the equilibrium between aqueous soluble and micellar bile salts during elution was avoided by preequilibration of the column with buffer containing the aqueous soluble concentration of the bile salt. Elution volumes were converted to size dimensions from a calibration curve produced from the elution volumes of proteins and small unilamellar vesicles of known size. Micelle sizes determined for several different lecithin-bile salt mixtures were consistent with those obtained by other techniques. The well-known hyperbolic increase in mixed micelle size as the lecithin to bile salt ratio approaches the micellar-vesicle phase limit was reproduced with this chromatographic technique. On the basis of these data and the recent observation by small-angle neutron scattering that lecithin-bile salt micelles increase in size by the elongation of constant-diameter rods [Hjelm et al. (1988) J. Appl. Crystallogr. 21, 858-863], a new model for the mixed micelle structure is proposed. According to this model, separation of the lecithin head groups by bile salts inserted along the rod surface produces a radial orientation of lecithin molecules along the length of the rod. Each end of the rod is sealed off by a lecithin-bile salt configuration that is richer in bile salts than the rod portion of the micelle. A simple mathematical description of this model predicts the observed changes in micellar size as a function of the lecithin-bile salt ratio with parameters that are consistent with lecithin and bile salt molecular dimensions.