A technique recommended for the assay of lipid and other organic peroxides based on the use of a commercial color reagent (El-Saadani et al., J. Lipid Res. 30, 627-630, 1989) has the advantage over other iodometric methods of being insensitive to oxygen. Although tested so far with a limited range of peroxides, this aerobic method has found popular use with complex biological systems, such as plasma. We have examined the ability of this assay to provide accurate estimates of peroxides in H2O2, tert-butanol, and cumene hydroperoxides, and in oxidized linoleate, low-density lipoprotein, and human blood plasma. The results were compared with values obtained with an anaerobic iodomeric peroxide method taken as the standard peroxide assay. We found that the published protocol gave correct peroxide values for H2O2 solutions. Correct values could also be obtained for oxidized low-density lipoprotein, provided that the incubation period was extended from 30 to 60 min. All the other peroxides tested gave much lower values than those of the standard iodometric method. Incubation at 50°C to increase the velocity of the reaction for some of the slowly reacting peroxides did not improve the accuracy of the aerobic method. We recommend that the color reagent should be used as originally specified only for the assay of H2O2, or for oxidized lipoprotein with the incubation extended to 60 min.