The objective of our studies is to define the molecular mechanisms that determine: (a) The chain length of the carbohydrate preassembled on the lipid carrier and the role of glucosyl residues in chain processing, (b) The structural factors in small peptides that define whether or not they will serve as a substrate for the oligosaccharide substrate, and (c) The participation of nascent chains on the RER preparations we have isolated as substrates in glycosylation.