Mice were fed diets containing 960 mg (control), 100 mg (moderately Mg deficient) and 30 mg (severely Mg deficient) of Mg/kg. After 20 days, mice were inoculated with Babesia hylomysci (from Dr Wery, Anvers, Belgium). Significant increases in RBC Mg levels were observed following infection. All the control and moderately deficient mice died from infection, whereas the severely Mg-deficient diet protected mice against infection, as shown by a decrease in parasitaemia and mortality. The decrease in RBC Mg, modifications in membrane properties and increased oxidant stress are possible explanations for the protective effect of severe Mg deficiency. When mice were maintained for 2 months after inoculation on a severely Mg-deficient diet and were then switched to a control diet, all survived and had low parasitaemias. After 1 month, these mice were rechallenged with B. hylomysci and 89% survived.