A particulate enzyme preparation from Cryptococcus neoformans transferred the mannosyl residue from GDP-mannose:o an acceptor consisting of a commercial preparation of methyl 3-O-α-mannopyranosyl-α-mannopyranoside (confining 10% 2-O-α-mannopyranosyl-α-mannopyranoside). The configuration of the new bond was alpha by its susceptibility to α-mannosidase; the amount of product was dependent on the concentration of enzyme, of GDP-mannose, and of acceptor. The optimal temperature and pH were 37 °C and 7.0, respectively. Manganous ion was required for activity and acetyl coenzyme A was stimulatory. Studies suggested that dolichyl phosphate intermediates were not involved in this mannose transfer. The fact that none of the several acapsular mutants tested were deficient in this mannosyltransferase suggested that this enzyme was not involved in synthesis of backbone mannan linkages in capsular polysaccharide. NMR analysis of the methylmannotriose product showed only α(1 → 2) linkages between sugar moieties. This mannosyltransferase evidently extends α(1 → 2) mannan by adding another α(1 → 2)-linked mannosyl residue. Its activity is appropriate for a role in synthesis of "high mannose" oligosaccharide moieties of glycoproteins.Key words: virulence, capsular polysaccharide, acapsular mutants, dolichol pathway, oligomannans.