SummaryHuman peripheral blood mononuclear cells were found to produce interferon (IFN) when stimulated by free P. falciparum parasites in vitro. On the other hand parasite‐infected, intact erythrocytes were unable to induce IFN synthesis. When the IFN was characterized according to sensitivity to anti‐IFN‐α antibodies and pH 2 treatment it was found to consist of IFN‐α. Cell fractionation procedures and analysis of each cell fraction with regard to natural killer (NK) cell activity and IFN‐producing capacity revealed that both activities were confined to the same cell fraction. The possible relevance of the IFN‐NK cell system in malaria