Haematological measurements were made in 198 Nigerian children aged three months to two years who received weekly malaria chemoprophylaxis with chloroquine from shortly after birth until the age of one or two years and in 185 age-matched control children. Children protected against malaria had a higher mean haemoglobin level and a higher packed cell volume than control children, and they showed fewer abnormalities of their red cells. Total and differential white blood cell counts, mean plasma folate and mean serum ferritin concentrations were similar in both groups of children. However, the geometric mean red cell folate level of children exposed to malaria was significantly higher than the mean level of control children; and it may be that malaria raises the red cell folate through intracellular synthesis by malaria parasites. Children with malaria parasitaemia had a significantly lower haemoglobin and packed cell volume and a significantly higher geometric mean red cell folate and ferritin level than children without parasitaemia. Serum ferritin is probably an unreliable index of iron status in children with malaria.