AbstractSome cases of occurrence of matrix effects (mostly ion suppression) in protein‐precipitated plasma samples, and their influence on the validity of plasma concentrations and pharmacokinetic parameters, are discussed. The comparison of matrix effects using either electrospray (TurboIonspray, TISP) or atmospheric pressure chemical ionization (APCI) indicated that APCI is less prone to matrix effects. Nevertheless, TISP is usually the first choice of ionization technique since unknown thermally labile metabolites might be present in the plasma samples causing erroneous results. A high impact of ion suppression on the plasma concentrations after intravenous (i.v.) administration was found, depending on the drug formulation (vehicle). Since ion suppression caused significantly lower plasma concentrations (by a factor of up to 5.5) after i.v. dosing, the area under the curve (AUC) was underestimated and the plasma clearance was consequently erroneously high, with an impact on drug candidate selection. By simple stepwise dilution (e.g. 10‐fold and 50‐fold) of the supernatant of protein‐precipitated plasma samples, including all calibration and quality control samples, the matrix effects were recognized and eliminated. Copyright © 2003 John Wiley & Sons, Ltd.