Abstract 1. 1. Trout liver microsomes were found to catalyze the transfer of mannose from GDP-mannose to glycoproteins and to two lipids which have been characterized. 2. 2. The lower phase in the Folch extraction procedure contained a mannolipid with properties of dolichyl mannosyl phosphate: it co-chromatographed on silica gel thin layer plates with authentic dolichyl mannosyl phosphate and was stable under weak alkaline conditions but yielded mannose upon acid hydrolysis. 3. 3. This mannolipid did not undergo hydrolysis upon incubation in presence of phenol as expected for non-allylic type of polyprenyl mannosyl phosphate; the mannolipid was made soluble in water upon ozonolysis; after this treatment the product was submitted to β-mannosidase which resulted in releasing mannose; under the same condition α-mannosidase is inactive. 4. 4. The other lipid has been characterized as lipid-P-P-oligosaccharide. 5. 5. The oligosaccharide moiety was a mixture ranging from a trisaccharide to a pentasaccharide made of one to three mannose units bound to a chitobiose unit. 6. 6. Another transfer has been found which led to the formation of mannoglycoproteins.