The product of the proto-oncogene c-kit is a transmembrane receptor protein that plays an important role in the regulation of normal and neoplastic hematopoiesis via the interaction with its specific ligand termed stem cell factor. To examine whether c-kit product is possibly involved in the pathogenesis of human lymphomas, we analyzed the expression of the c-kit protein in neoplastic cells from a variety of lymphoid tumors by immunostaining of lymph node frozen sections with the 17F11 antibody, detecting an extracellular epitope of the c-kit receptor, and of c-kit RNA by Northern blot hybridization. Of 24 nonHodgkin's lymphomas (NHL) of B- and T-cell phenotype, none expressed immunodetectable c-kit protein that was also not evidenced in lymphoid cells of reactive lymph nodes and normal tonsils. In contrast, c-kit protein was expressed by Reed-Sternberg cells and their mononuclear variants from 11 of 21 Hodgkin's disease (HD) cases, and in tumor cells from 11 of 16 cases of CD30+ anaplastic large cell lymphoma (ALCL). c-kit specific mRNA was also detected in lymph node tissues from HD and ALCL cases but not in neoplastic tissues from NHL other than ALCL. In addition, c-kit/CD30+ tumor cells were evidenced by flow cytometry in a patient displaying massive bone marrow involvement by ALCL. With the exclusion of lymphocyte predominance cases of HD that resulted c-kit expression and the other histologic subtypes of HD or the immunologic phenotype of tumor cells (B, T, nonB-nonT) in both HD and ALCL. The highly restricted expression of the c-kit product among human lymphomas to HD and ALCL provides a further biologic link between these two closely related lymphoma entities.