The antigenic formula and chemical structure of capsular polysaccharide (CPS) of Cryptococcus albidus var. albidus (C. albidus) were studied in relation to those of C. neoformans var. neoformans serotype A (C. neoformans A). The results of slide agglutination tests with factor sera and reciprocal adsorption experiments showed that antigenic formula of C. albidus was the same as that of C. neoformans A. The soluble CPSs from the two species were obtained from culture supernatants by precipitation with ethanol followed by purification by chromatography on DEAE-cellulose column. The structural analyses of such CPSs from the two species showed that the antigenic CPS fractions consisted of a backbone of alpha(1-3)-linked D-mannopyranosyl residues with a single branch of beta(1-2)-xylose or glucuronic acid, and mostly with O-acetyl groups, in which side chains and O-acetyl groups were responsible for antigenic specificity. It was found that there was a minor difference between the CPS of C. neoformans A and that of C. albidus; in the former, unsubstituted mannose residues existed in a low frequency, but in the latter none. Moreover, the 1H-nuclear magnetic resonance spectra of partially hydrolyzed acidic fragments of the two CPSs indicated that two xylose side chains were present between glucuronic acid side chains. Taken together, it was suggested that these two species of C. neoformans A and C. albidus are closely related to each other in their CPSs.