The selectivity of size-exclusion chromatog. (SEC) can be modified by adding non-ionic micelles to the mobile phase. Surfactant-aided size-exclusion chromatog. (SASEC) can therefore very well be performed in a gradient mode on an SMB, as is reported in this paper. A method has been developed for correctly positioning a micellar gradient over an SMB. The method is applied for size-exclusion chromatog. with the non-ionic surfactant C12E23 as gradient forming solute, and demonstrated by applying it to a relevant chromatog. protein sepn. problem.