cDNA isolated from a human lymphocyte library were analyzed and shown to encode a novel cell-surface glycoprotein, termed HB15, expressed by dendritic cell subsets and activated lymphocytes. The predicted mature 186 amino acid protein was composed of a single extracellular V-type Ig-like domain, a transmembrane region, and a 39-amino acid cytoplasmic domain. In contrast to most Ig-like domains, analysis of a partial genomic DNA clone revealed that the extracellular Ig-like domain of HB15 was encoded by at least two exons. Northern blot analysis revealed that HB15 derived from three mRNA transcripts of approximately 1.7, 2.0, and 2.5 kb expressed by lymphoblastoid cell lines. Two mAb reactive with HB15 were produced and used to show that HB15 is expressed as a single chain cell-surface glycoprotein of M(r) 45,000. HB15 expression was specific for lymphoblastoid cell lines and mitogen-activated lymphocytes, and HB15 was not expressed at detectable levels by circulating leukocytes. Immunohistologic analysis revealed that HB15 had a unique pattern of expression, being found predominantly in hemopoietic tissues with strong expression by scattered interfollicular interdigitating reticulum cells and weak expression by germinal center cells. HB15 was also expressed by Langerhans cells within the skin. HB15 therefore serves as a unique marker for the subset of dendritic cells represented by Langerhans cells and interdigitating reticulum cells. Thus, the HB15 glycoprotein represents a newly identified member of the Ig superfamily that may play a significant role in Ag presentation or the cellular interactions that follow lymphocyte activation.