Three auxin analogs, 4-, 5-, and 6-azido-3-indoleacetic acid (4-N(3)-IAA, 5-N(3)-IAA, and 6-N(3)-IAA) have been synthesized for use as fluorescent photoaffinity labeling agents. The pK(a) values of these compounds (4-N(3)-IAA, 4.67; 5-N(3)-IAA, 4.65; 6-N(3)-IAA, 4.66; all +/- 0.04) are experimentally indistinguishable from the pK(a) of 3-indoleacetic acid (IAA, 4.69 +/- 0.04). The auxin activity of these IAA derivatives has been determined in several systems. In soybean, pea, and corn straight growth assays, all three analogs induce growth comparable to that caused by IAA. In the tobacco pith assay, all three analogs elicit a maximum increase in fresh weight at least 40 to 50% of that caused by IAA. Optimal growth is attained in the tobacco pith assay at slightly higher concentrations of 4-N(3)-IAA and 6-N(3)-IAA (30 micromolar) than required for IAA (10 micromolar); however, maximal growth is achieved at a slightly lower concentration of 5-N(3)-IAA (3 micromolar). The N(3)-IAAs, like IAA, are transported basipetally through tobacco pith tissue.