Some quassinoids (1-6) isolated previously as plant growth inhibitors from the leaves of Eurycoma longifolia Jack. (Simaroubaceae) were subjected to in vitro tests on anti-tumor promoting, antischistosomal and plasmodicidal activities. The most active compound for inhibition of tumor promoter-induced Epstein-Barr virus activation (anti-tumor promotion) was 14,15beta-dihydroxyklaineanone (5, IC(50) = 5 microM). Longilactone (1) gave significant antischistosomal effect at a concentration of 200 microg/ml. 11-Dehydroklaineanone (3) and 15beta-O-acetyl-14-hydroxyklaineanone (6) showed potent plasmodicidal activity (IC(50) = 2 microg/ml). Thus it was suggested that E. longifolia possesses high medicinal values due to the occurrence of a variety of quassinoids. Copyright 2002 Elsevier Science Ireland Ltd.