A method for monitoring the biological activity of broth polysaccharides of Agaricus blazei (AB-BP) in a submerged culture is described. The TNF-alpha releasing capability of AB-BP on the macrophage cell line, RAW 264.7, correlated with the molecular weight of AB-BP. The quality of polysaccharide in a submerged culture of Agaricus blazei was indirectly monitored by analyzing the distribution of its molecular weight within 1 h. The harvest time of the maximum polysaccharide production did not coincide with that of the maximum biological characteristics in the batch culture.