Serum antibodies to Candida albicans were determined in 36 sera from 33 “high risk” patients by precipitation with cytoplasmic candidal extracts (“S” antigen), by agglutination, and by indirect immunofluorescence methods. In nine patients who proved free of candidal infection at autopsy, precipitin tests were negative, and agglutinin and fluorescent antibody titers ranged from 0 to 1:320. The precipitin test was positive in 21 (89%) of 24 patients in whom systemic candidiasis was confirmed by autopsy or during life, or both: 17 (70%) of these had agglutinin, and 16 (67%) had fluorescent antibody titers of 1:320 or higher. For reasons discussed in the text, the precipitin reaction is considered diagnostically more specific for systemic candidiasis than the other two responses studied. Conflicting reports concerning “false positive” and “false negative” precipitin reactions are discussed. It is shown that prolonged storage of sera may result in a loss of 75 to 100% of precipitating antibody, so that retrospective evaluations of this test may yield a deceptively high proportion of “false negative” reactions.