Interleukin (IL)-1α is primarily translated as a 33 kDa molecule (IL-1α_1–271), and then processed into a 17 kDa molecule (IL-1α_119–271) by calpain. The precursor region of IL-1α (IL-1α_1–118) contains a nuclear localization signal (KVLKKRRL, residues 79–86). We investigated the intracellular localization IL-1α fused with green fluorescent protein or β-galactosidase. IL-1α_1–118 was localized exclusively in nucleus, but IL-1α_1–271 in both the nucleus and the cytoplasm, suggesting the presence of a cytoplasmic retention signal within the mature region of IL-1α. Furthermore, the intracellular localization of IL-1α with deletions from the C terminus, internal deletions and point mutations suggested that the cytoplasmic retention signal is located within residues 168–201.