Ship is a recently identified SH2-containing inositol polyphosphate 5-phosphatase that has been implicated as an important signaling molecule in cell-culture systems. To understand the physiologic function of Ship in vivo, we performed expression studies of Ship during mouse development. Results of this study demonstrate the expression of ship to be in late primitive-streak stage embryos (7.5 days postcoitus [dpc]), when hematopoiesis is thought to begin, and the expression is restricted to the hematopoietic lineage in mouse embryo. In adult mice, Ship expression continues to be in the majority of cells from hematopoietic origin, including granulocytes, monocytes, and lymphocytes, and is also found in the spermatids of the testis. Furthermore, the level of Ship expression is developmentally regulated during T-cell maturation. These results suggest a possible role for Ship in the differentiation and maintenance of the hematopoietic lineages and in spermatogenesis.