To investigate the influence of Plasmodium falciparum malaria on plasma antioxidants and lipid peroxidation, plasma ascorbate, urate, total protein and albumin, ceruloplasmin and malondialdehyde (MDA) were determined in two groups of 42 patients each, one with mild and the other with severe falciparum malaria, and in an equal number of age- and sex-matched control subjects. Plasma MDA was found to be significantly higher in malaria patients, and the increase was proportional to the severity of the disease. Of the antioxidants, ascorbate and albumin decreased with severity of disease while urate and ceruloplasmin increased. Only ascorbate correlated inversely with MDA both in mild (r = -0.341, P < 0.05) and severe malaria (r = -0.545, P < 0.01). While plasma albumin correlated inversely (r = -0.442, P < 0.01), urate and ceruloplasmin correlated directly (r = 0.419, P < 0.01 and r = 0.349, P < 0.05, respectively) only in patients with severe malaria. These antioxidants also correlated well with markers of disease severity, indicating the influence of disease severity in regulating their levels in plasma. The presence of significant quantities of ascorbate and albumin, along with increases in some of the other antioxidants and MDA, indicates ineffectiveness of the antioxidant defense system in controlling plasma lipid peroxide content. Increased amounts of thiobarbituric acid-reactive material could have been the result of spillover from increased tissue peroxidation or the presence of pro-oxidants in malarial plasma.