Previous studies indicate a suppressive influence of fish oils on rodent malaria. The present work was carried out to study (i) the dose-effect relation between dietary fish oils and lethality of primary malaria infection in mice; (ii) the modifying influence of vitamin E; and (iii) the effect of previous fish oil feeding on parasitemia and lethality of a rechallenge infection. For two or four weeks, groups of weanling male mice were fed a standard laboratory diet or one of eight purified diets containing various amounts of fish oil (providing 6-21% of energy). The diets were prepared with and without vitamin E. After the two- or four-week feeding period, the mice were injected intraperitoneally with Plasmodium yoelii-infected erythrocytes. Six months after the primary infection (four months after discontinuing fish oil feeding), the surviving mice were again injected intraperitoneally with parasitized red blood cells (or even better--erythrocytes, erythrocytes are used elsewhere). Primary malaria infection was lethal in mice fed standard diet alone or with fish oil and vitamin E added. In contrast, feeding a fish oil-based diet without vitamin E improved survival to at least 70% if the mice had been fed these diets for four weeks. Protection against malaria did not seem to be related to the fish oil dose used. Regardless of the previous fish oil dose, all the mice surviving the primary infection survived the rechallenge infection with low parasitaemias.(ABSTRACT TRUNCATED AT 250 WORDS)