The effects of irradiation with β (10 MeV fast electrons)- and γ-rays were studied on several characters in strains of the cultured mushroom under different physiological and environmental conditions, including uncut and cut mushrooms, tightness of packing, and relative humidity. Weight loss was greatest in the non-irradiated mushrooms owing to evaporation from an increased surface area resulting from expansion and ripening which were greatly retarded in the irradiated samples. Twenty-five krads of β- or γ-rays had a significant, but transitory, effect on the veil opening. The inhibition became long-lasting and improved with increasing dose. High relative humidity and free exchange of gases were essential for maintaining quality. Depending on the conditions, expansion of pilei and opening of veils were affected differently by irradiation with β- and γ-rays. The strains displayed different opening rates. Expansion and elongation were retarded significantly by 100 krads. The effect improved further with increasing dose. Irradiation improved the skin colour when the mushrooms were stored uncovered or in boxes with perforated PVC-foil. The opposite was the case when the boxes were sealed. The colour of the flesh was examined by measuring its reflectance. Edaphic factors or age of the culture (flush-number) influenced the slope of the reflectance curves. Generally, irradiation increased flesh discoloration, but more in the stipes than in pilei. This adverse effect would make acceptance of irradiated mushrooms by consumers questionable. The flesh of the pilei was almost unaffected by the packing, while packing gave a positive response on the stipes. Surface mould fluff was reduced by 50 krads of β- or γ-rays and almost extinct after 200-krad treatment. Growth of pathogens such as Pseudomonas tolaasii and Mycogone pemiciosa was prevented by 200 krads.