Ferrofluid-modified trypsin has been used for the detection and determination of selected xenobiotics that inhibit trypsin activity. The procedure is useful especially when colored samples or samples containing suspended solid impurities are to be assayed. Ferrofluid-modified trypsin was inhibited by Ag+ and Pb2+, selected dyes (safranin, thionin), bacitracin and 4-aminobenzamidine. Enzymes immobilized on magnetic particles can form a basis of new automated assay procedures for the determination of xenobiotics.