The effect of ethylenediamine tetracetic acid (EDTA) on endogenous Mn2+ was studied by electron spin resonance (ESR) spectroscopy during indoleacetic acid (IAA)-induced growth of Avena coleoptile segments. An enhancement in the concentration of endogenous Mn2+ was recorded in coleoptile segments treated with low concentrations (1 × 10−6 M) of EDTA or ethyleneglycol-bis-(β-aminoethylether) N, N″-tetracetic acid (EGTA - 1 × 10−6 to 1 × 10−5 M), while at higher concentrations both EDTA and EGTA appeared to chelate Mn2+. An intermediate concentration of EDTA (5 × 10−5 M) interacted with IAA in enhancing the growth, while both lower (1 × 10−6M) and higher (5 × 10−6M) concentrations inhibited IAA-induced growth. Inhibition of IAA-induced growth at higher concentrations of EDTA was reversed by exogenous Mn2+. Concentration of endogenous Mn2+ declined further when IAA was present along with different concentrations of EDTA, except in coleoptile segments treated with 5 × 10−4 M EDTA. The data suggest that endogenous Mn2+ is essential for IAA-induced growth and that a precise balance of free to bound/complexed Mn2+ may be involved in IAA action.