CD68 is a 110-Kd transmembrane glycoprotein of unknown function highly expressed by human monocytes and tissue macrophages. We have isolated cDNA clones encoding CD68 from a U937 cDNA library by transient expression in COS cells and panning with the anti-CD68 monoclonal antibodies (MoAbs) Y2/131, Y1/82A, EBM11, and Ki-M6. CD68 transcripts are constitutively present in the promonocyte cell line U937 and are upregulated by phorbol myristic acid (PMA). By contrast, CD68 transcripts are absent or present at very low levels in many hematopoietic lines including KG1, CEM, and K562, but can be induced by exposure to PMA. The cDNA sequence predicts a type I integral membrane protein of 354 residues with a heavily glycosylated extracellular domain of 298 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 proline hinge: a membrane-distal mucin-like domain containing short peptide repeats and consisting of 54% serine and threonine residues; and a membrane proximal domain that has significant sequence homology to a family of lysosomal/plasma membrane shuttling proteins known as the lamp 1 group. CD68 is a member of a growing family of hematopoietic mucin-like molecules, including leukosialin/CD43, the stem cell antigen CD34, and the lymph node high endothelial ligand for L-selectin GlyCAM-1.