The competitive binding specificities of glucuronoxylomannan (GXM) and its derivatives to factor sera of Cryptococcus neoformans were studied by enzyme-linked immunosorbent assay. An effort was made to determine the epitope specificity of each factor serum. Despite the presence of antigenic factor 1 on all serotypes of C. neoformans, variations in inhibition ability were observed with different GXMs. The panspecific component of factor serum 1 (antibody 1) appeared to be due to the presence of more than one antibody component. The activity was dependent on the 6-O-acetyl substituent. GXMs of serotypes A and D inhibited factor serum 2 equally well, indicating a low titer for the antibody 7 component. Serotype B GXM was a poor inhibitor, and serotype C GXM did not inhibit factor serum 2. The activity of factor serum 2 was 6-O-acetyl dependent. GXMs from typical serotype A and serotype D isolates were excellent inhibitors of factor serum 3. GXMs from serotype B were poorly inhibitory and serotype C did not inhibit factor serum 3. The activity of factor serum 3 was 6-O-acetyl dependent. The activity of factor serum 4 was due predominantly to antibody component 6. The activity of factor 4 was directed mainly against serotype C, and it was independent of 6-O-acetyl substitution Factor serum 5 was specific for serotype B GXMs. The inhibitory effect was independent of 6-O-acetyl substitution, but the effect was diminished by reduction of the glucuronic acid. The GXMs with a typical serotype C structure inhibited antibody 6. O deacetylation of the GXMs did not affect their inhibitory activity. However, reduction of glucuronic acid reduced factor serum 6 binding. Factor serum 8 was specific to serotype D; native GXMs of serotype A were slightly inhibitory. O deacetylation of the serotype D GXMs abrogated the inhibitory effect. O deacetylation alone abrogates the activity of antibody components 1, 2, 3, and 8. Reduction of glucuronic acid reduces the inhibitory activity of the GXM to antibody components 4, 5, and 6. Partial GXM structures and methyl glycosides did not effectively inhibit the activity of any of the factor sera.