SummaryFour preparations of purified mannans from C. albicans Berkhout, one purified mannan from S. cerevisiae, a lipid fraction from C. albicans Berkhout, and eight polysaccharide-protein complexes from the same microorganism were tested for interferon-stimulating activity in explanted mouse peritoneal leukocytes. Whereas the mannan from S. cerevisiae was inactive, the purified mannans as well as the polysaccharide-protein complexes from C. albicans with a molecular weight around 20,000 regularly demonstrated an ability to stimulate interferon release. Glucan-protein complexes were inactive. No correlation between N, P, and nucleic acid content of preparation and its interferon-stimulating activity was found. Ribonuclease or hydroxylamine treatment did not alter the activity of these preparations. A lipoprotein fraction from C. albicans examined as a potential contaminant showed a low degree of activity. It was concluded that it is the mannan itself which stimulated the interferon production from leukocy...