There is growing interest in measuring the antioxidant status of plant tissues. This protocol describes the oxygen radical absorbance capacity (ORAC) assay, which measures antioxidant inhibition of peroxyl radical-induced oxidations and is a measure of total antioxidant capacity. The assay is performed in a microplate and is assessed with a 96-well multi-detection plate reader. Total antioxidant capacity of 64 experimental samples can easily be analyzed in 1 d. This assay is presented along with rapid assays for total phenolic content and total ascorbate content. Overall, these assays provide a general diagnostic tool of the antioxidant capacity in leaf tissue extracts.