Biological therapies are claiming a place in the routine management of some solid tumours. In this review we focus on the biological treatment of melanoma and renal-cell carcinoma, identifying the background to current practice and areas of promise that may be in routine clinical use in the near future. Melanomas and renal-cell carcinomas are particularly resistant to chemotherapy and radiotherapy and are characterised by the host immune response to the tumours. For this reason there has been particular interest in the biological therapy of these diseases. Biological therapies differ from chemotherapeutic approaches in their mechanism of action, time to response, and side-effect profiles. Although biological treatment has a long history, it is only with recent advances in immunology and molecular biology that progress has been made. In the next few years investigators expect to build on their research experience with biotherapeutic agents to provide tangible benefits for patients.