Proteinase Inhibitor I was induced to accumulate in tobacco (Nicotiana tabaccum) leaves by placing plants in darkness for 10 days at 27 degrees C. The inhibitor was isolated using ammonium sulfate precipitation, Sephadex G-75 chromatography, heating, and affinity chromatography with a chymotrypsin-Sepharose column. Inhibitor I was purified 232-fold with a yield of 34 mg from 2.5 kg of leaves. Affinity-purified tobacco Inhibitor I was shown to be homogeneous by gel electrophoresis in both nondissociating and dissociating buffers. The inhibitor has a molecular weight of 39,000 +/- 1000 determined by gel filtration and, like its potato and tomato counterparts, is composed of five subunits of molecular weight 8100. The tobacco Inhibitor I strongly inhibits chymotrypsin and weakly inhibits trypsin. The chemical, physical, and immunological properties of tobacco Inhibitor I indicate that it is structurally very similar to potato tuber Inhibitor I and tomato leaf Inhibitor I, although the synthesis and accumulation of the three inhibitors in their respective tissues are all under different developmental or environmental regulation.