Titers of serum IgG with specificity for C. albicans mannan were assessed by enzyme-linked immunosorbent assay (ELISA) in a group of 640 normal persons. Substantial differences in titers were found when comparing individuals but not among groups divided by race, sex, or age. As a group, leukemics showed a pattern of IgG titers similar to that of the normal population. Sera from six patients with chronic mucocutaneous candidiasis showed that IgG levels were markedly variable in this disease. Anti-mannan IgG was not detected in normal rabbit serum but was found in serum from infected animals. The results indicate the necessity of serial sampling if serological methods, such as ELISA, are to be useful in aiding the diagnosis of invasive candidiasis.