Various species of mushrooms belonging to genus Pleurotus (Fr.) Kumm. are known by its nutritional value and medicinal properties, particularly anti-tumor activity and immunomodulation, related to structural polysaccharides. In order to study the composition and the effects of aqueoussoluble constituents from Pleurotus ostreato-roseus Sing. on mice serum proteins, the P. ostreato-roseus strain POR-020 was cultured on powered grass-sugarcane bagasse-sawdust-wheat bran-substrate, and fruiting bodies were produced, milled, extracted with ethanol and hot water, and fractional precipitated with ethanol producing two fractions, FI-1 and FI-2. These fractions showed, respectively, 47.87 and 26.45% (d. m.) carbohydrate, and 2.07 and 1.97% (d.m.) protein composition. The fractions were administered i.p. in AJ male mice in a singledose of 257 mg/kg (FI-1) and 31.5 mg/kg (FI-2). After 15 days administration, electrophoretic analysis of serum proteins detected that FI-1 increased 50, 25, 0.35, and 28.6% in b-, a1-, a2- and g-globulin regions, and FI-2 increased 4.4 and 21 % in band a1-globulin, respectively.