A flow system has been developed for a fast and convenient assay of LD-1 and total LD in serum. The LD-1 is separated immunochemically and subsequently determined amperometrically by a thin layer transducer in a flow system. Potassium ferricyanide was used as a mediator at a potential of 0.07 V vs Ag/AgCl to eliminate any protein fouling of the electrode. A kinetic measurement helps to diminish several interferences in the electrode response. Thus, it is possible to determine the LD-1 or LD-1/total LD conveniently and accurately even at low activities.