Serine proteinase activity was assayed during sporophore maturation and postharvest senescence in Agaricus bisporus. Serine proteinase accumulated largely in stipe tissue early during postharvest storage and in the later stages of maturation of unpicked sporophores. Western blot staining correlated with enzyme activity indicating de novo synthesis. The enzyme could not be detected by Western blotting in freshly harvested sporophores at an early maturation stage. Small increases in activity were detected in the cap tissue during the final stages of senescence, postharvest and in unpicked sporophores. The distribution of the serine proteinase in senescent sporophore tissues (high in stipe, low in cap) was confirmed by histochemical and immunologically stained tissue prints.