SDS-PAGE of clathrin coated vesicle (CCV) fractions prepared from developing pea cotyledons are characterized by the presence of a 28 kDa polypeptide. Like clathrin light chains, this polypeptide is heat-stable and can bind calcium ions. However, the distribution of this polypeptide is not identical to that of clathrin in the rate zonal gradients used to purify CCV. Negatively stained preparations reveal small, 12 nm diameter hollow particles, in addition to CCV. As judged by the electron dense centre to these particles, we infer that CCV preparations from pea cotyledons are contaminated with phytoferritin. This has been confirmed by immuno-blotting with pea ferritin antibodies. The degree of phytoferritin contamination in CCV fractions is greater when RNase digestion of postmicrosomal pellets is performed at cold, rather than warm temperatures.