Sainfoin (Onobrychis viciifolia Scop.) organs and tissues were surveyed for the presence of proanthocyanidins or condensed tannin (CT) in an attempt to learn more about CT formation. Three histological methods were used: a vanillin–HCl stain to localize areas of CT formation through a distinctive cherry-red coloration of CT-containing cells; osmium fixation to further localize CT formation in cellular organelles using light microscopy; and fine localization and examination using electron microscopy. Cells containing CTs were found in relatively large numbers throughout the sainfoin plant in all organs except the roots. No CTs were found in the cotyledons or in some petals. Ultrastructural studies revealed that CTs are restricted to the vacuoles and appear to accumulate on the inner periphery adjacent to the tonoplast or in the centre of the organelle, possibly next to vacuolar invaginations. Key words: condensed tannin, proanthocyanidin, sainfoin, histology, transmission electron microscopy.