Published studies support the hypothesis that at least two antigens of Candida albicans and Candida tropicalis circulate in the bloodstream of patients with severe candidiasis. One antigen (probably mannan) is stabile and the other (probably protein) is labile. The stabile antigen can rarely be detected without prior dissociation from antibody. Dissociation treatment destroys the labile antigen, leaving in doubt whether any is antibody bound. Dissociation steps have also been necessary for detection of Aspergillus fumigatus antigen in sera from patients with invasive aspergillosis. Concentrations of the stabile antigens of both Candida and Aspergillus appear to be in the nanogram-per-milliliter range, generally lying at the limits of detection by conventional assays. Improvements in sensitivity and practicality are needed, but the tests are clearly promising.