Fenozan B07, a difluorinated 3,3'-spirocyclopentane, 1,2,4-trioxane, is a novel, second-generation antimalarial endoperoxide which is a potent blood schizontocide against strains of rodent malaria that are highly resistant to a wide spectrum of classical antimalarials. Like compounds of the artemisinin series, its action is limited to the intra-erythrocytic stages, both asexual and sexual, and it is devoid of causal prophylactic activity. Both Fenozan B07 and the artemisinins are potent gametocytocides. In contrast to arteether, in a model using synchronous infection with Plasmodium vinckei petteri, Fenozan B07 inhibits the development of all asexual stages except preschizonts, as well as gametocytes. The activity of the artemisinin series in rodent-malaria models is limited to the rings and young trophozoites. The combined effect of Fenozan B07 with artesunate against P. v. petteri was only additive. A slight degree of potentiation was found in mice infected with asynchronous, drug-sensitive P. berghei but the combination was only additive against CQ-resistant P. yoelli ssp. NS. On the other hand, a significant degree of synergism was observed when mice infected with the artemisinin-resistant ART line of P. yoelii ssp. NS received combinations of Fenozan B07 with artemisinin. The conclusion is drawn from these and other data that there are significant differences between the blood schizontocidal actions of Fenozan B07 and the artemisinins. The basis of these differences remains to be determined.