Disulphide interchange can be demonstrated as follows. To a solution of oxidised glutathione a sufficient amount of acetic anhydride is added to obtain complete conversion of oxidised glutathione. After completion of the acetylation a fresh sample of oxidised glutathione is added. If the mixture of oxidised glutathione and diacetyl oxidised glutathione thus obtained is incubated, disulphide interchange occurs if a proper catalyst has been added. This interchange is evident from the appearance of monoacetyl oxidised glutathione. The three compounds involved in this reaction (oxidised glutathione, monoacetyl and diacetyl oxidised glutathione) were readily distinguished by paper electrophoresis at pH 1.7. Using this method several compounds were tested for their catalysing or inhibiting effect upon disulphide interchange. SH-containing compounds catalyse the reaction, in conformity with literature data; the SH-generating compounds Na2SO3 and KCN also catalyse the reaction, the latter compound being far less efficient than the former one. It was shown by amperometric titration that slow spontaneous disulphide interchange is due to traces of SH compound in the oxidised glutathione preparation. SH-blocking compounds inhibit this spontaneous interchange.