An extracellular glycoprotein exhibiting immune activity was produced by Trichosporon pullulans yeast grown in a defined ethanol-containing medium. The glycoprotein had a molecular mass of 100-150 kDa. The carbohydrate portion of the glycoprotein consisted of mannose as a predominant component, glucose, and a small amount of arabinose. This glycoprotein was named MAG. Immunological studies showed that MAG exhibited a high immunomodulating activity, revealed distinct adjuvant properties,had a wide therapeutic index, and increased non-specific resistance of mammals to infection. In batch culture, the most active MAG production (36 4 mg l(-1)) coincided with the phase of active yeast growth. In a chemostat under nitrogen limitation, the rate of the MAG synthesis increased with increasing a specific growth rate.