Polyadenylated RNA isolated from the endosperm tissue of maturing castor bean seeds was translated in a cell-free rabbit reticulocyte lysate system. Rabbit antibodies raised against Ricinus communis agglutinin were used to identify nascent agglutinin chains. In contrast to the authentic agglutinin polypeptides with molecular weights of 31000 (A chains) and 37000 (glycosylated B chains), immunoreactive translational products of Mr 33500 and 59000 were observed. The inclusion of canine pancreatic microsomes in the translational system resulted in the cotranslational segregation of these immunoreactive products into the lumen of the vesicles and their modification, to molecular weights of 32000 and 66000--69000 respectively. These cotranslational size modifications resulted from the cleavage of leader sequences and, in the case of the larger product, concomittant core glycosylation, 32000-Mr and 66000--69000-Mr proteins were also observed amongst the immunoreactive products initially formed during the labelling of intact endosperm tissue in vivo, together with 37000-Mr and 39000-Mr proteins. Pulse-chase experiments showed that 66000--69000-Mr proteins slowly disappeared while the smaller proteins were further cleaved to chains of Mr 31000 (authentic A chain), 34000 and 37000 (authentic glycosylated B chains). It was concluded that R. communis agglutinin polypeptides were synthesized in precursor form, possibly as a 'giant' precursor in the case of the B chain, on membrane-bound polysomes. Cotranslational translocation across the endoplasmic reticulum membrane was accompanied by proteolysis to remove leader sequences and, where appropriate, core glycosylation. Even after cotranslational processing agglutinin polypeptides were still in precursor form. Processing to authentic size appeared to occur posttranslationally.