An oligosaccharide-P-P-lipid has been isolated from porcine liver by extraction with organic solvents and purified by chromatography on silica gel and DEAE-cellulose. The purified oligosaccharide-lipid was shown to contain mannose and N-acetylglucosamine in an approximate ratio of 1:1 and our results suggest that the major oligosaccharide component in the preparation was a tetrasaccharide with the composition (Man)2 (GlcNAc)2. When the oligosaccharide-lipid was incubated with GDP-[14C]mannose and a solubilized enzyme preparation from rabbit liver in the presence of MgCl2, three radioactive products could be isolated. The oligosaccharides in the products could be identified as a penta-, a hexa-, and a heptasaccharide. These products were formed by the stepwise addition of mannose to the growing oligosaccharide chain and GDP-mannose was indicated as the glycosyl donor in each reaction.