A stability study for amodiaquine (AQ) and desethylamodiaquine (AQm) in whole blood and plasma is reported. AQ, AQm and chloroquine (CQ) were simultaneously analysed and the ratios AQ/CQ and AQm/CQ were used to ensure correct interpretation of the stability results. CQ was stable in whole blood and plasma at all tested temperatures enabling it to be a stability marker in stability studies. Simultaneous analysis of compounds, of which at least one is already known to be stable, permits a within sample ratio to be used as a stability indicator, The new approach significantly reduced bias when compared to the traditional approach. AQ and AQm were stable in plasma at -86 degrees C and -20 degrees C for 35 days, at 4 degrees C for 14 days and at 22 degrees C for 1 day. AQ and AQm were stable in blood at -86 degrees C and 4 degrees C for 35 days, at -20 degrees C and 22 degrees C for 7 days and at 37 degrees C for 1 day.