The murine T1 gene encodes a membrane-bound glycoprotein (T1-M), highly similar to interleukin-1 (IL-1) receptor type I, and a soluble variant (T1-S) representing its isolated extracellular domain. In vivo, the expression pattern of both T1 isoforms differs drastically. The T1-M receptor is abundantly expressed in single cells of the major hemopoietic organs (embryonic liver, spleen, bone marrow). It is restricted to few hemopoietic cell types throughout ontogenesis. By contrast, the soluble T1-S protein is predominantly expressed in selected nonhemopoietic embryonic tissues (developing skin, bone, and retina) and deposited in extracellular matrix. Despite the similarity of the T1 ligand-binding domain to all IL-1-binding proteins, it does not exhibit affinity to either IL-1 alpha or -beta. Thus, T1-M likely represents a novel orphan receptor of selected hemopoietic cells. The matrix-associated T1-S variant might act to create a reservoir of the putative T1 ligand in some differentiating tissues.