Interleukin(IL)-1 differs from most other cytokines in its lack of a signal sequence. This results in intracellular retention of the immature proform. The release of IL-1 has been shown to be restricted predominantly to activated monocytes and macrophages and to be associated with apoptosis of the producer cell. These features have limited the investigation of IL-1 in early immune responses. In order to study the biological effects of local IL-1 beta release during an antitumour immune response, we used B16 mouse melanoma cells transduced with mature human IL-1 beta cDNA constructs. To obtain a released form of human IL-1 beta (ssIL-1 beta), the signal sequence from the related IL-1 receptor antagonist was ligated to the cDNA that encoded the mature form of IL-1 beta. When cells of the poorly immunogenic B16 melanoma cell line were transduced with IL-1 beta by retroviral infection, high levels of the protein were detected intracellularly, whereas cells transduced with IL-1 beta containing the signal sequence secreted most of their protein. The in vitro growth of the melanoma cells was unaffected by the IL-1 beta or ssIL-1 beta gene transfer. In contrast, the in vivo subcutaneous tumour growth of the ssIL-1 beta-transduced B16 cells in syngeneic C57BL/6 mice was significantly reduced compared with the IL-1 beta- and the mock-transduced controls. Immunohistochemical analysis revealed the infiltration of macrophages to be strong in B16/ssIL-1 beta, moderate in B16/IL-1 beta and minimal in control tumours. Furthermore, a moderate infiltration of CD4+ cells and of scattered dendritic cells was detected in B16/ssIL-1 beta tumours whereas very few or no CD4+ cells and dendritic cells were seen in the B16/IL-1 beta or control tumours. Following in vivo growth, all the tumours upregulated ICAM-1 on their cell surfaces. However, the percentage of ICAM-1-expressing cells was two- to four-fold higher in B16/ssIL-1 beta tumours compared to the control. The data suggest that IL-1 beta acts in vivo, either directly or indirectly, as a chemotactic factor for monocytes, T helper cells and dendritic cells. This supports IL-1 beta having a regulatory effect on tumour growth when locally released in the tumour area.