The incidence of serum antinuclear antibodies and serum antibodies to single stranded (ss) and double stranded (ds) DNA was investigated following acute malaria in 58 Caucasians visiting tropical countries but resident in Britain and in 24 Ghanaians resident in Ghana. In Caucasians this infection was associated with a significant increase in the incidence of speckled antinuclear antibodies (38% compared to 3% in controls; P less than 0.001) and a significant rise in antibody levels against ssDNA (14% compared to 5%; P less than 0.05), but no rise in antibodies against dsDNA. Acute malaria in Ghanaians was associated with an incidence of 25% of antinuclear antibodies and 4% of antibodies to ssDNA; these were similar to those found in healthy Ghanaians who are chronically exposed to malaria. Antibodies against dsDNA were not detected. The incidence of antinuclear antibodies and levels of anti-ssDNA antibodies was higher in the Ghanaian healthy population than in normal Caucasians. These observations indicate that malaria is associated with the development of antinuclear and anti-ssDNA antibodies. Ghanaian patients with a tropical splenomegaly syndrome or with a nephrotic syndrome, both of which conditions are suspected of having a malarial aetiology, had serum levels of anti-ssDNA higher than healthy controls. This observation adds further circumstantial evidence to the role of malaria in causing anti-DNA antibodies.