Interleukin (IL)-12, produced by mononuclear phagocytes, activates the T-helper 1 (Th1) cells and helps, as a mediator, the innate immune response to intracellular microbes. In Plasmodium falciparum infection, this proinflammatory cytokine has immunoregulatory functions with effects on the immune response to the blood stage of disease, but also induces protection and reduces malarial anaemia. In this study, the levels of IL-12 were determined in 73 African children, aged 2-144 months (median 19.5 months), who had severe or mild P. falciparum malaria. IL-12 was determined using the enzyme-linked immunosorbent assay. The levels of IL-12 were found to be significantly elevated (21.6 +/- 18.3 pg/ml) in patients who suffered less severely from the disease. In contrast, the levels of IL-12 were found to be lower (13.1 +/- 7.11 pg/ml) in patients who suffered more severely from the disease.