To study the mechanism of accumulation of radiocesium by the mushroom (Pleurotus ostreatus (Fr.) Kummer Y-1), uptake of137 Cs and Cs in the presence of the alkali elements was investigated. The concentration ratio (concentration in the mycelia or the fruit body (dry)/concentration in the medium (wet); CR) of137Cs was almost identical with that of Cs. The CR of137Cs decreased with increasing of Cs, K or Rb concentration in the media. Similarly, the CR of K decreased with increasing the Cs concentration in the media. The result of137Cs gel filtration chromatography of the mycelia cultivated in the media containing Cs showed that137Cs was not bound to molecules with more than 10000 molecular weight.