The extraction, purification, and isolation of the growth inhibitor previously postulated are described. Methanol extraction and separation into acid, neutral, and basic fractions was followed by paper chromatography of the acid and neutral fractions with distilled water, re-extraction with methanol, and thin-layer chromatography, the peak of inhibition being located at Rf 0.7–0.8 (isopropanol: ammonia: water, 100:5:5), or Rf 0.3–0.4 (chloroform: ethyl acetate: acetic acid, 60:40:5) Lunularia gemmae, grown directly on the chromatographic strip with added nutrient solution, served as the most appropriate and direct bioassay. Area measurements after 5–10 days' growth yielded significant differences. Other bioassays included: Marchantia polymorpha gemmae, lettuce hypocotyl growth, cress-seed germination, oat coleoptile, and radish cotyledon disc tests. An active inhibitor, i.e. dihydrohydrangeic acid, now named ‘lunularic acid’, was isolated in crystalline form. Lunularic acid was found to increase with long-day treatment of Lunularia thalli, though present even in short-day. Its concentration could be altered rapidly when daylength conditions were changed. The growth inhibition was linearly related to concentration over the range from 0.1 to 10 ppm, very high concentrations being lethal. Abscisic acid, though inhibitory to Lunularia in low concentrations, was not detected in extracts, and could easily be separated from lunularic acid.