To examine the expression of Ifi 200 genes in vivo and add new information about their function, polyclonal monospecific rabbit antibodies, designated N-term or C-term, were raised against both the N-terminus and C-terminus of the 204 protein (p204) respectively. Western blotting analysis demonstrated that p204 and D3, another member of the Ifi 200 gene family, are constitutively expressed, though at different degrees, in bone marrow, thymus and lymph nodes, and barely detectable in the spleen. Poly rI:rC treatment did not modulate their expression. Peritoneal resident macrophages (Mphi) from untreated mice were negative, but displayed high levels of both p204 and D3 on poly rI:rC treatment. A significant increase of these proteins is also observed when Mphi are cultured overnight in vitro with IFNs or LPS. Lung, kidney and brain were negative for p204 and D3 expression. These results, together with immunohistochemical analysis, demonstrate that the 204 gene has an expression pattern restricted to cells of the myelomonocytic lineage similar to that observed for the human homolog, the myeloid nuclear differentiation antigen (MNDA) suggesting its potential involvement in the differentiation and maturation of this cell lineage.