A method was developed to remove continuously the concentration gradient of any low-molecular-mass substance in the eluate during adsorption liquid column chromatography. For this purpose, a hollow-fibre membrane module (HFMM) was directly connected to the outlet of the chromatographic column and on-line buffer exchange was achieved by cross-flow ultradialysis. It could be demonstrated that the HFMM when used during liquid chromatography has some additional advantages. It is an ideal tool to concentrate continuously column eluates by dialysis against polyethylene glycol solutions. Moreover, it is also possible to detect specifically enzyme activities on-line using an HFMM.