The suitability of the vacancy affinity capillary electrophoretic method (VACE) for study of displacement of a target drug from a protein by simultaneously administered drugs was investigated. As test system, the displacement of warfarin from bovine serum albumin (BSA) by furosemide and phenylbutazone was selected, It appears that the displacement can be observed well from the shift of the actual mobility of warfarin when a displacer drug is added. Also, the competitive action of the displacer drugs (affinity for BSA) is clearly visible. The VACE method seems to be attractive for rapid assesssment of information about the competitive properties of coadministered compounds. (C) 1998 Elsevier Science B.V. All rights reserved.