The accumulating evidence that erythropoietin and erythropoietin receptor are expressed in various non-haematopoietic organs suggests that erythropoietin signalling might be involved in the growth of tumours, but this possibility has never been examined. We found that mRNAs for erythropoietin and erythropoietin receptor are expressed in malignant tumours of female reproductive organs, where erythropoietin levels are higher than in normal tissues. Furthermore, tumour cells and capillary endothelium showed erythropoietin receptor immunoreactivity. To investigate the role of the erythropoietin/erythropoietin receptor pathway in these tumours, we injected mouse monoclonal antibody against erythropoietin or the soluble form of erythropoietin receptor into blocks of tumour specimens and cultured the blocks. After 12 h of injections, these blocks were examined and compared with control blocks injected with mouse monoclonal antibody, heat denatured soluble form of erythropoietin receptor, mouse serum or saline. Tumour cells and capillaries were markedly decreased in a dose-dependent manner after either injection. A marked increase of the cells containing fragmented DNA and the histopathological characteristics of these cells suggest that the decrease in tumour cells and capillary endothelial cells was due to apoptotic cell death. The co-existence of JAK2 and phosphorylated-JAK2, and STAT5 and phosphorylated STAT5, all of which are involved in the mitogenic signalling of erythropoietin, was found frequently in tumour cells and capillary endothelial cells in the untreated blocks. In contrast, most of the phosphorylated-JAK2- or phosphorylated-STAT5-positive cells had disappeared in the experimental blocks. Moreover, reduced tyrosine phosphorylation of STAT5 in the experimental blocks was confirmed by western blotting analysis. The results strongly indicate that erythropoietin signalling contributes to the growth and/or survival of both transformed cells and capillary endothelial cells in these tumours. Thus, deprivation of erythropoietin signalling may be a useful therapy for erythropoietin-producing malignant tumours.