Human granulocyte-macrophage colony stimulating factor (hGM-CSF) is used clinically to treat leucopenia typically caused by cancer chemotherapy or radiotherapy. This study used multiple strategies to obtain very high expression levels of OsrhGM-CSF (14 mu g/seed) in rice endosperm. Electron micrographs of immunogold-labeled transgenic endosperm showed that rhGM-CSF was not only localized in protein bodies but was also distributed in the apoplast. A biological activity assay indicated that OsrhGM-CSF stimulated the growth of TF-1 cells in vitro. In addition, the transgene was used to effectively treat leucopenia by oral administration of the unprocessed transgenic grains. In cyclophosphamide-induced leucopenic mice, transgenic seeds produced a 27% (t = 0.021) gain in leukocytes after 14 days feeding. Even in non-leucopenic mice, leukocyte gain was 37% (t = 0.002) more than that of mice fed non-transgenic seeds. This study provides a novel approach to the use of oral unprocessed transgenic OsrhGM-CSF seeds to treat leucopenia.