Oligomeric proanthocyanidins were isolated from rhatany root (Krameria triandra) and characterised by acid hydrolysis, thiolytic degradation, and spectroscopic methods. They consisted of 2-14 flavanol units with mainly 2,3-cis configuration and with a propelagonidin:procyanidin ratio of 65:35. The predominant interflavan linkage was [4,8]. [4,6]-Bonds were present in the higher oligomers, presumably forming branched chain units. The astringency of the drug was mainly due to proanthocyanidins with degrees of polymerization from 5-10, and these were also the astringent compounds of rhatany tea and tincture. A pharmacological screening of the extract revealed a significant antimicrobial effect.