Summary Chloroplast destined precursor proteins bind to distinct areas on the organellar surface as visualized by immunogold decoration. Binding seems to occur predominantly in regions where the space between outer and inner envelope membranes appears dense in electron microscopic pictures. When complete protein translocation is blocked by antibodies, which bind specifically to the precursor protein, this precursor can still be partially translocated into the organell, as deduced from its maturation by the stromal processing peptidase. It remains, however, sensitive to exogenous protease, thus indicating that the preprotein spans both membranes while in transit through the plastid envelopes. Chloroplast envelope polypeptides that are involved and in close proximity to the precursor protein in the translocation event are identified by chemical crosslinking. Crosslinking experiments, using different translocation systems, i.e. intact organelles, outer envelope vesicles and an isolated import complex, gave identical labelled products. These crosslinked polypeptides are likely candidates for active participants in the import process.