Experiments were conducted to evaluate the effect of Fe3+-chelate concentration and chemical properties on the rate of reduction by roots of Fe-deficient peanut plants (Arachis hypogea L. cv. Florigiant). Reduction studies with the Fe3+-chelates of ethylenediaminetetraacetate (EDTA), hydroxyethylethylenediaminetriacetate (HEDTA), jV,yV-ethylenediaminediacetate, and diethylenetriaminepentaacetate (DTPA) each showed saturation kinetics characteristic of active processes by plants. Km values varied from 23 to 85 pM, and Kmax from 0-55 to 5-5 pmol h-1 (g fresh roots)-1. Fmax values declined as the Fe3+-chelate formation constant increased. The removal of plant shoots reduced initial (< 6 h) reduction rates by 22%. The Km' for Fe3+-chelates was much higher than Fe3+-chelate concentrations in soil solutions of problem calcareous soils.