The application of a continuous-flow dialysis system, consisting of a membrane dialyser and a trace enrichment column, in on-line combination with tandem mass spectrometry via a thermospray interface is described. The method is applied to the quantitation of drugs in complex biological matrices containing macromolecular interferences. The potential of the method is demonstrated by the quantitative analysis of the anti-cancer drug rogletimide in the plasma of patients after treatment.