The fluorometric thiobarbituric acid (TBA) assay of blood plasma was performed under various conditions in order to assess whether the assay reflects lipid peroxidation. The TBA-reactivity of malonaldehyde was not dependent on the pH values of the reaction and was little affected by tert-butyl hydroperoxide (tert-BuOOH). The reactivity of 2,4-nonadienal and 2-hexenal, which was maximal at pH 3-4 and at above pH 4, respectively, was dramatically enhanced by tert-BuOOH and ferric ion, and suppressed by ethylenediaminetetraacetic acid (EDTA). The pigment formation from oxidized low-density-lipoprotein was maximal at about pH 5, markedly enhanced by ferric ion and suppressed by EDTA, indicating that the TBA-reactive substances in the oxidized lipoprotein were composed of malonaldehyde, alkadienals and alkenals originated from lipid peroxidation. The pigment formation from plasma and its phosphotungstic acid precipitate was maximal at pH 4 and at below pH 2, respectively. The effect of tert-BuOOH, ferric ion and EDTA was not significant. The major TBA-reactive substances from plasma may be compounds other than malonaldehyde, alkadienals, alkenals and those in the oxidized lipoprotein. The TBA-reactivity of plasma does not appear to be specific to lipid peroxidation.