The expression of endogenous interleukin-3 (IL-3) gene in mouse bone marrow after whole body irradiation was assessed by immunocytochemistry, in situ hybridization (ISH), and in situ reverse transcription PCR (IS RT-PCR). The results showed obvious injury to the bone marrow followed by recovery 4 weeks after radiation. IL-3 protein was increased in the cytoplasm of recovering hematopoietic cells, especially at 21 days after radiation, while the mRNA was negative, except at 15 days when the mRNA was weakly positive by ISH. IS RT-PCR showed that IL-3 mRNA was strongly positive in the cytoplasm of recovering hematopoietic cells, especially at 10 to 15 days after radiation. We postulate that endogenous IL-3 gene may accelerate hematopoietic recovery in irradiated bone marrow and that IS RT-PCR may objectively report the regulation of IL-3 gene expression.