Assays have been developed for some transfer reactions involved in the synthesis of Saccharomyces cerevisiae wall mannoproteins, both in a particulate preparation in the presence of EDTA or Triton X-100, and after lipid extraction with chloroform-methanol at -20 C. The mannosyl transferase activities were also studied in cells made permeable to GDP-mannose by toluene-ethanol treatment ("in situ"). In these permeabilized cells, the glycosylating reactions dependent on lipid carriers (dolichol derivatives) did not function, but those independent of them were unaffected. The lipid-independent mannosyl transferase activities were partially inhibited by nucleotide diphosphates probably in a competitive manner. Increase of the nucleotide diphosphate pool "in vivo" might slow down the speed of the transfer reactions carried out by the mannan synthetase system.