Interleukin 1 alpha (IL-1 alpha) is synthesized as a 33 kDa form and proteolytically processed into a 17 kDa form. Although IL-1 alpha has no signal peptide, it is released from cells. To investigate the relationship between the processing and release of IL-1 alpha, human bladder carcinoma cells (HTB9 5637) which express IL-1 alpha constitutively, were treated with calcium ionophore (A23187). A23187 induced the processing of 33 kDa IL-1 alpha and selectively released processed 17 kDa IL-1 alpha, without any change in the release of 33 kDa IL-1 alpha. When extracellular calcium was chelated by EGTA, or when intracellular calpain was inhibited by the cell-permeable cysteine-protease inhibitor, E64d, the processing of 33 kDa IL-1 alpha was significantly blocked, the release of 33 kDa IL-1 alpha being unchanged. These results indicate that the release of IL-1 alpha was accompanied by the processing of 33 kDa IL-1 alpha.