Cryo-substituted spinach leaf pieces, embedded in LR-White resin by chemical polymerization at low temperature, were used to localize enzymes within chloroplasts by immuno-electron microscopy. Employing monospecific antibodies and protein A-gold as label, the spatial distribution of six chloroplast enzymes was investigated. Statistical methods were used to determine whether each enzyme was bound to the thylakoids. By this means the coupling factor 1 (CF1), ferredoxin-NADP+ reductase, sedoheptulosebisphosphatase, and ribulose-5-phosphate kinase were found to be membrane-associated. In case of glyceraldehyde-3-phosphate dehydrogenase the label was only weak and an unequivocal determination of its location was not possible. In contrast, ribulosebisphosphate carboxylase was randomly distributed throughout the chloroplast.