There is now clear evidence that cells cultured from human and animal tumours can be induced to differentiate in vitro by recognised hormones, regulatory peptides, polar solvents and cytotoxic drugs. Examples can be found from several different types of tumour with the bulk of the data deriving from neuroblastoma and myeloid leukaemia. There is no clear correlation of inducer with cell type, other than some specific peptides like MSH, and agents such as dimethyl sulphoxide and dexamethasone have wide ranging activity. Steroid hormone action may require interaction between different cell types, and the inability of tumours to differentiate in situ may implicate reduced cell-cell interaction, possibly due to degradation of extracellular matrix, or to alteration of the stromal phenotype by tumour-derived factors such as peptides or prostaglandins. When differentiation has been demonstrated, it has been possible, in some cases, to correlate increased differentiation with reduced malignancy by in vitro characterisation or tumorigenicity. Conditions which induce differentiation in rat mammary carcinoma and mouse myeloma also reduce tumour growth in vivo. Clinical trials have not provided any conclusive evidence for a therapeutic benefit so far, but relatively few trials have been carried out. There is clearly a need for further investigation both in vitro and in vivo to select optimal conditions and combinations of agents for clinical evaluation.