<jats:title>Abstract</jats:title><jats:p>Methanol and chloroform extracts were prepared from various parts of four plants collected in Mali: <jats:italic>Guiera senegalensis</jats:italic> (Gmel.) Combretaceae, <jats:italic>Feretia apodanthera</jats:italic> (Del.) Rubiaceae, <jats:italic>Combretum micranthum</jats:italic> (Don.) Combretaceae, <jats:italic>Securidaca longepedunculata</jats:italic> (Fres.) Polygalaceae and two plants collected in Sao Tome: <jats:italic>Pycnanthus angolensis</jats:italic> (Welw.) Myristicaceae and <jats:italic>Morinda citrifolia</jats:italic> (Benth.) Rubiaceae were assessed for their <jats:italic>in vitro</jats:italic> antimalarial activity and their cytotoxic effects on human monocytes (THP1 cells) by flow cytometry. The methanol extract of leaves of <jats:italic>Feretia apodanthera</jats:italic> and the chloroform extract of roots of <jats:italic>Guiera senegalensis</jats:italic> exhibited a pronounced antimalarial activity. Two alkaloids isolated from the active extract of <jats:italic>Guiera senegalensis</jats:italic>, harman and tetrahydroharman, showed antimalarial activity (IC<jats:sub>50</jats:sub> lower than 4 μg/mL) and displayed low toxicity against THP1. Moreover, the decrease of THP1 cells in S phase of the cell cycle, after treatment with harman and tetrahydroharman, was probably due to an inhibition of total protein synthesis. Copyright © 2002 John Wiley & Sons, Ltd.