The biomass of 18 strains of Agaricus subrufescens and of 13 strains of Agaricus bisporus was chemically analyzed using solid-state (13)C NMR. The study focused on polysaccharides because they can play a major role as antitumor molecules. The study also examined whether biomass chemical properties varied between the vegetative mycelium and the fruiting bodies of A. subrufescens, and these data were compared with the mycelium of A. bisporus. Qualitative differences between vegetative mycelia and fruiting bodies were observed, whereas quantitative differences were measured between the two species with a higher percentage of polysaccharides in the biomass of A. subrufescens. This Agaricus species is thus an interesting potential source of polysaccharides with medicinal properties, both from vegetative mycelium obtained in liquid cultures and from fruiting bodies produced on composts.