Investigation of in vivo bone marrow stimulating effect of Agaricuss bisporus (White bottom Mushroom, WBM). Water extract of white button mushroom was prepared using soxlet apparatus. Myelosuppression in wistar rats induced by busulfan induce bone marrow suppression model. Water extract of white button mushroom 250 mg/kg, p.o. was given to wistar rats for 21 days. At the end of treatment Hb, RBC, total WBC, neutrophil, basophile, eosinophil, lymphocyte, monocyte where measured using automated blood cells counter. Toxic dose of busulfan induced sever bone marrow depression in disease control and treatment control groups. There were significant increase in hemoglobin, red blood cell, differential white blood cell (WBC) and total platelet count in rats treated with water extract of WBM compared to disease control group rats. Beta glucan, antioxidant, protocatechuic acid and pyrocatechol present in water extract of WBM may be responsible for immunostimulating activity in busulfan induced bone marrow suppression model in wistar rats.