The initiation of carbohydrate prosthetic group attachment onto liver glycoprotein apopolypeptides occurs while the latter are still bound nascent to their sites of synthesis on endoplasmic reticulum-bound ribosomes. Free ribosomes or free polysomes appear to be inactive in glycoprotein initiation. Though initial incorporation of glucosamine-14c is fastest into agranular reticulum glycoproteins, the granular reticulum soon attains a similar specific activity by 20-30 min. after an intraperitoneal injection of glucosamine. The most active, early-labeled components of the agranular fraction are low-density membranes that differ from plasma membrane fragments and the Dallner subtractions of agranular reticulum. || ABSTRACT AUTHORS: Authors