VARIOUS immunological changes have been observed in many apparently healthy subjects living in tropical regions that are probably related to multiple parasitic infections from childhood¹. Consideration of this in conjunction with the rarity of autoimmune disease in tropical Africa, for example Western Nigeria², led to the suggestion that the immunological effects of repeated parasitic infection might have some protective action against the development of autoimmune disease². To investigate this hypothesis we have studied the effects of infection with malaria on the spontaneous autoimmune disease of NZB mice and the (NZB × NZW)F₁ hybrid mice (B/W mice). Twenty-one NZB mice and thirteen female B/W mice from the Taplow colony were infected, when one month old, with the rodent malaria parasite Plasmodium berghei yoelii³ by the intraperitoneal injection of 1 × 10⁶ parasites derived from a heavily infected mouse of the same strain. The malaria strain was not contaminated with Eperythrozoon coccoides. Parasitaemia was monitored by the examination of Giemsa stained blood films made from the tails of the mice every second day during the acute phase of the infection and at longer intervals thereafter. To determine whether low parasitaemia was persisting, heparinized blood samples were collected from each mouse 4 months after infection and pooled samples injected into young BALB/c mice.