Differentiation of tissue macrophages from bone marrow progenitor cells via circulating blood monocytes is a complex and multistep process. In order to analyze macrophage maturation on the molecular level we used the method of mRNA differential display to identify novel genes that are preferentially expressed in mature, in vitro differentiated macrophages. A PCR-fragment could be isolated that was amplified from macrophages, but not from freshly isolated monocytes, and macrophage-specific expression was confirmed by Northern blot analysis. Several corresponding cDNA clones could be isolated from a macrophage cDNA library, covering a nucleotide sequence of 2.5 kb. Nucleotide sequence of this cDNA showed no homology to known genes. The sequence reveals an open reading frame that codes for a 238 amino acid hydrophobic protein with seven potential transmembrane domains, suggesting a possible role as a receptor or an ion channel.