Specific labeling of the plasma membrane of intact protoplasts from C. albicans, using lactoperoxidase-catalyzed iodination, has permitted the development of a procedure for isolating relatively pure preparations of this component. The specific activity of the labeled membrane was very low, indicating that only a few proteins are exposed on the outer membrane surface. The specific activity of labeling increased almost 100-fold when both membrane surfaces were exposed to iodination. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in combination with autoradiography, indicated that only 2 glycosylated proteins are exposed on the outer membrane surface, whilst all membrane proteins can be labeled in isolated plasma membrane preparations. Extraction of mannan-protein from purified cell walls of C. albicans gave material which, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, resembled the exposed plasma membrane glycoproteins. Purified plasma membranes incorporated mannose from GDPmannose on to an endogenous protein acceptor. The addition of exogenous cell wall mannan-protein increased the degree of incorporation. Approximately 12% of the incorporated mannose was sensitive to .beta.-elimination, but a lipid intermediate was not involved in the reaction. An Arrhenius plot of enzyme activity at different temperatures showed a discontinuity at 17.degree. C. The mannan synthetase activity was also significantly less at 40.degree. C than at 37.degree. C, temperatures at which the yeast-mycelial transition was found to occur in this organism. The possible biosynthetic relationship between the exposed glycoproteins of the plasma membrane and those of the cell wall is discussed.