Existing colorimetric methods for the estimation of indoleacetic acid (IAA) were evaluated. Type of complexing metal, acid, and reagent ratios were studied as to effect on color intensity and stability. Recommended is an iron-perchloric acid procedure of increased sensitivity and specificity: To a 1 ml. aliquot add 2 ml. of reagent (1 ml. 0.5 FeCl3 plus 50 ml. 35% HClO4); read after 25 min. at 530 m[mu]; suggested range[long dash]0.2 to 45 [mu]g. IAA/ml. The stability of the color formed allows a large number of detns. to be made at one time without adherence to a fixed time schedule. Presented for the various methods are absorbancy/IAA concn. curves and a table of relative specificities toward other 3-substituted indoles.