Serum lipid peroxides were measured by a thiobarbituric acid (TBA) method. TBA color reactions carried out under the conditions with acetic acid (TBA-AA) or hydrochloric acid (TBA-HCl) systems, and then TBA value was detected by colorimetric ([OD]) and fluorometric ([FR]) measurements. Effects of ferrous ion, ferric ion and EDTA on the both systems were examined. Equal TBA value dependent upon concentrations of MDA (tetraethoxypropane) were obtained on both systems detected by [OD], but the relative fluorescence of TBA-AA system was higher than that of TBA-HCl system. Fluorometric determination was suitable for low TBA value, and colorimetric one was for high TBA value. Good correlation between TBA[OD] values and TBA[FR] values of serum from patients was estimated, but it was suggested that effects of serum substance were differ on the both systems. Effects of iron on TBA-AA and TBA-HCl systems were different significantly, particularly high Fe-TBA values obtained from TBA-AA system in the presence of ferrous ion. The fact suggests that serum lipid peroxidation is induced by ferrous ion during a heating of TBA color reaction. Same effect of ferric ion in TBA-AA system was observed. The increment of TBA values by ferrous and ferric ions was inhibited by a addition of EDTA. It was considered that the difference effect of iron on the both systems was due to pH and acetic acid concentration. The relationship of TBA values and Fe-TBA values of serum from patients was no correlated, and the various inhibition ratios of EDTA for the TBA value was calculated. It is suggested that Fe-TBA value is possible to detemine the qualitative difference of serum lipid peroxides.