A new secreted yeast glycoprotein with an M(r) of about 400 kDa (gp400) has been found. The glycoprotein is an O-mannosylated oligomer, whose synthesis and export into culture medium are stimulated by heat shock. Intracellular transport of gp400 is carried out by membrane vesicles distinct from the known constitutive secretory vesicles. Immunological analysis revealed gp400 only in Saccharomyces species.