In the presence of Mn'+ and uridine diphosphate-N-acetyl-D-['4C]glucosamine, a total particulate fraction prepared from Dictyostelium discoideum amoebae catalyzed the transfer of N-acetyl['4C]glucosamine to endogenous membrane protein acceptors. No transfer to lipid acceptors was evident. The '4C products obtained from growth-phase and aggregation-competent amoebae were converted to glycopeptides by pronase digestion. The respective glycopeptides appeared identical in their chemical and chromatographic properties, suggesting that the same activity was functioning in both growing and differentiating cells. The results provided no evidence for developmental regulation of this activity in D. discoideum.