Aqueous extract of air-dried lambsquarters (Chenopodium album) at 25 mg/ml significantly inhibited germination and growth of radish and wheat seeds. Soybean seed germination was not inhibited; however, hypocotyl growth was significantly reduced. Germination of radish seeds in sand amended with pulverized lambsquarters shoots at 2 and 4 mg/g was reduced 40 and 95%, respectively. Shoot dry weight and plant height were also reduced 30 and 9%, respectively, at 4 mg/g, but not at 2 mg/g concentration. Residues after extraction with water incorporated in sand were not inhibitory, indicating water solubility of the inhibitor(s). Aqueous extract of shoots decomposed for five days lost nearly 40% of its inhibitory effect; 20% of it still persisted in the extract of shoots decomposed for 30 days. The filtrate from ultrafiltration of aqueous extract through a pad of molecular-weight cutoff 1000 inhibited radish seeds germination and growth, indicating that the molecular weight of the inhibitor(s) was less than 1000. Partitioning of the aqueous extract by a series of solvents resulted in isolation of an inhibitor(s) in the butanol fraction. Seven phenolics were identified in this fraction using high-performance liquid chromatography (HPLC). Paper chromatographic analysis of the butanol fraction revealed six bands, of which one band withR f =0.83 inhibited germination and growth of radish seeds. Chlorogenic acid identified by HPLC appeared to be the principal component of the phytotoxin.