Macrosialin is a heavily glycosylated transmembrane protein of 87-115 kDa, highly and specifically expressed by mouse tissue macrophages, and to a lesser extent by dendritic cells. We have isolated cDNA clones encoding macrosialin from a thioglycollate-elicited peritoneal macrophage cDNA library by transient expression in COS cells and panning with the anti-macrosialin monoclonal antibody FA/11. A single 1.3-kilobase macrosialin transcript was detected in both untreated and phorbol 12-myristate 13-acetate-stimulated RAW cells. The cDNA sequence predicts a type I integral membrane protein of 326 residues with a heavily glycosylated extracellular domain of 306 residues containing nine potential N-linked glycosylation sites and numerous potential O-linked glycosylation sites. The extracellular domain consists of two distinct regions, separated by an extended 12 residue proline-rich hinge; a membrane-distal mucin-like domain of 89 residues containing short peptide repeats and consisting of 44% serine and threonine residues; and a membrane proximal domain of 170 residues, which has significant sequence homology to a family of lysosomal associated glycoproteins known as the lamp-1 group. Macrosialin is the murine homologue of the human macrophage glycoprotein CD68 (72% identity, 80% similarity). Both proteins are preferentially expressed by macrophages and share the same bipartite structure having a mucin-like domain and a domain common to the lamp family. Macrosialin and CD68 are the first examples of a lamp family protein with a restricted cell-type-specific expression. They may have evolved from the lamps to carry out specialized functions in dedicated phagocytic cells.