Historically, the process of developing new anticancer agents was largely empirical. Today, because of improvements in our knowledge of the molecular processes involved in the development of cancer, the process of developing new agents is becoming more rational. Researchers from Cancer Research UK, the European Organisation for Research and Treatment of Cancer and the National Cancer Institute have shown that, by undertaking a pharmacological approach to the selection of potential anticancer agents, both meaningful antitumour data and an 80% reduction in animal usage can be obtained. It has also been demonstrated that a new pharmacological tool, the "hollow fibre system", in which tumour cells are grown in biocompatible fibres which are implanted into mice, can be used to produce meaningful antitumour data with pharmacodynamic endpoints. By increasing the amount of data that can be obtained from a single animal and opening up the possibility of eliminating the need for untreated control animals, the hollow fibre system has the potential to make a significant contribution to both reduction and refinement.