Aldehydes generated during radical-induced lipid peroxidation, in particular 4-hydroxynonenal, are known to inhibit growth of certain cells. To extend our arguments that free radicals might be involved in the host response against malaria parasites we tested 26 carbonyls (n-alkanals, C6-C11; 2-alkenals, C3-C9; 2,4-alkadienals, C7, C9, C10; 4-OH-2-alkenals, C6, C8, C9; 2-alkanones, C3-C9; and malonyldialdehyde) against Plasmodium falciparum in vitro. We had previously detected many of these substances in oxidant-stressed, malaria-infected erythrocytes. Three 2,4-alkadienals (C7, C9 and C10) and three 4-OH-2-alkenals (C6, C8 and C9), at 20-100 microM concentrations, markedly inhibited incorporation of [3H]-hypoxanthine by P. falciparum. Acrolein had low effect, and none of the other compounds (12 aldehydes and 7 ketones) were active at concentrations up to 100 microM. Malonyldialdehyde was without effect at concentrations up to 450 microM. The aldehydes found to be inhibitory against P. falciparum could contribute to both the non-antibody host responses against this parasite and the antimalarial effects of radical-generating compounds such as t-butyl hydroperoxide, hydrogen peroxide, alloxan, isouramil, divicine and primaquine.