Antiretroviral protease inhibitors were assessed in vitro for their activity against Giardia duodenalis and Trichomonas vaginalis. Kaletra (a co-formulation of ritonavir and lopinavir) was the most effective overall, with 50% effective drug concentrations (EC(50)) of 1.1-2.7 microM (ritonavir concentration) against G. duodenalis and 6.8-8 microM against metronidazole-sensitive and clinically metronidazole-resistant T. vaginalis. Minimal inhibitory concentrations were 2-2.5 microM and 10-50 microM for G. duodenalis and T. vaginalis, respectively. Within the range of human plasma concentrations for ritonavir, only G. duodenalis was inhibited. Lopinavir alone was less inhibitory than ritonavir but was associated with a blockage in cytokinesis of G. duodenalis trophozoites. Saquinavir was not effective. These findings are significant considering the association between human immunodeficiency virus and T. vaginalis, and between G. duodenalis and homosexual behaviour.