Agaricus bisporus fruiting bodies were subjected to processing: blanching, boiling in water and blanching followed by lactic acid fermentation. Water soluble polysaccharides were obtained from fruiting bodies, quantified and analysed in terms of chemical composition, antioxidant capacity and antiproliferative activity towards cancer cell lines (MCF-7 and T-47D). The content of water soluble polysaccharides ranged from 7.35 ± 0.12 to 6.90 ± 0.17 mg g−1 fresh weight of sample. Boiling in water and blanching followed by fermenting caused minor decrease in the content. Fourier Transform Infrared spectroscopy of the extracted polysaccharides showed that both α- and β-linkages are present in the samples. Size exclusion chromatography confirmed the presence of 163.3 and 1.9 kDa molecules. The processing caused changes in chemical composition (protein and phenolics content decreased). The isolated polysaccharides exerted antioxidant and antiproliferative activities. Both the activities were slightly lowered as the result of the processing.