A barley cDNA clone encoding a putative cysteine protease with sequence homology to cathepsin B from mammalian cells has been characterized. This barley gene (CatB) is ubiquitously expressed, its mRNA being detected in leaves and roots, immature, mature and germinating embryos, in developing endosperms, and in aleurones upon germination, as assessed by northern blot analysis. The CatB mRNA expression in leaves increased by cold shock (6 degrees C), was not affected by wounding, and was under circadian control. These transcripts increased in the aleurone upon germination, whereas those for a cystatin encoding gene (Icy), that inhibits commercial cathepsin B in vitro, decreased. Gibberellin (GA) treatment of isolated barley aleurones induced and abscisic acid (ABA) repressed the steady-state levels of CatB mRNA, while Icy expression had an opposite pattern of mRNA accumulation in aleurones treated with GA. No response to GA or ABA was detected in leaves.