3-Methyleneoxindole (MO), a metabolite of the plant auxin 1- H-indole-3-acetic acid (IAA), is a potent sulfhydryl reagent that can profoundly affect bacterial growth and metabolism. For investigative purposes, MO is obtained from the degradation of 3-bromooxindole-3-acetic acid (3-Br-lAA) in aqueous media. Alternatively, it can be prepared from the riboflavin-catalyzed photooxidation of IAA. My earlier claims that MO possesses auxin activity were refuted by independent investigators either because the results could not be reproduced when 3-Br-IAA was used, or the results were ascribed to contamination with residual IAA if MO obtained from photooxidation was used. Recent investigations indicate that, contrary to previous assumptions, the quantitative degradation of 3-Br-lAA resulting in the formation of MO is not instantaneous; depending on the purity of 3-Br-lAA, it may take several hours to several days to reach completion. Furthermore, aqueous solutions of MO ≥0.1 mm are rapidly polymerized, thus causing a loss of biological activity. These findings may explain why MO that is derived from 3-Br-lAA often fails to produce auxin action. Ultrapure MO, obtained from either 3-Br-IAA or photooxidation, is 50- to 1000-fold as effective as IAA in the straight growth assay, induction of xylogenesis in parenchymatous tissue, and rooting of explants in tissue culture.