Both polysomes and polysomal RNA, isolated from cotyledons of ripening pea (Pisum sativum) seeds and supplemented respectively with wheat germ S-100 and S-30 fractions, were used to program the cell-free synthesis of polypeptides. The relationship of these polypeptide products to seed storage proteins has been investigated. When fractionated on sucrose density gradients the translation products did not coincide with native storage proteins, nor were they exactly coincident with the subunits of storage proteins on dissociating gels. Treatment with antiserum prepared against storage proteins precipitated only a very small proportion of these products. Nonetheless, tryptic peptide mapping showed that a significant proportion (up to 65%) of the in vitro products from cell-free systems were related to the storage proteins. Alternative interpretations of these results are that either the translatable mRNAs for storage proteins make up a small proportion of the total template isolated from pea cotyledon polysomes, or that storage protein polypeptides are made in significant amounts in vitro but lack major antigenic determinants which in vivo may be acquired during chain completion or post-translational modification.