A crude organelle preparation from germinating castor bean endosperm catalysed the incorporation of mannose from GDP[(14)C]mannose into acid-labile mannolipids. Solubility and chromatographic properties have identified the most rapidly synthesized products as mannosyl-phosphoryl-polyisoprenol, while the more polar lipid formed was shown to contain oligosaccharide. Little radioactivity from GDP[(14)C]mannose accumulated in insoluble product in the cell-free system, but supplying GDP[(14)C]mannose to intact endosperm tissue has shown that the major incorporation product in vivo is glycoprotein. This product was readily solubilized by either pronase or sodium dodecyl sulphate treatment suggesting it was membrane bound glycoprotein. Incorporation of mannose into mannosyl-phosphoryl-polyisoprenol during the cell-free assay was stimulated by the addition of dolichol monophosphate. This enzymic activity was optimal at pH 7.5 and in the presence of 10 mM Mg(2+). The Km for GDP-mannose was estimated to be 5×10(-7) M. Cellular mannosyl transferase activity changed markedly during early post-germinative growth; from being absent in the dry seed, enzyme activity increased to peak between the second and third days of growth and subsequently declined.