We have made transgenic mice using the human erythropoietin receptor (hEpoR) encoding gene contained within a 15-kb DNA fragment. The transgenic mice that incorporated the hEpoR transgene into the genome were analyzed for tissue specific expression of hEpoR mRNA using reverse transcriptase and DNA amplification. In the control animals, endogenous EpoR transcripts were identified in bone marrow and spleen; no transcripts were detected in heart, kidney, liver, or brain. In the transgenic mice, hEpoR transcripts were detected in bone marrow and spleen but not in heart, kidney, or liver, suggesting that the transgene contains sufficient genetic information to direct appropriate expression in hematopoietically active tissues. The hematological parameters of the transgenic mice were within normal limits, consistent with the relatively low level of hEpoR transcripts detected. Surprisingly, hEpoR transcripts but not mouse EpoR transcripts were detected in the brains of the transgenic mice. Brain hEpoR transcripts were observed in all transgenic mice assayed, indicating that transgene expression in the brain did not result from the effects of aberrant integration sites. Comparable expression of the transgene was also observed in the embryonic brain. Interestingly, we observed significant expression of the endogenous EpoR gene in the early embryonic brain (Day 10) of normal mice at levels comparable to that observed in the adult spleen and bone marrow. The level of endogenous EpoR expression in the brain decreased during embryonic development to nondetectable levels prior to birth preceding the decrease of endogenous EpoR expression in the fetal liver, while hEpoR expression in the brains of transgenic mice persisted throughout embryonic development into adulthood. These data suggest that the hEpoR transgene contains appropriate regulatory sequences to direct tissue specific expression in tissues associated with hematopoietic activity and in the embryonic brain, but lacks the control elements to provide levels of expression comparable to that of the endogenous gene or to selectively silence brain expression in the adult mouse.