A neoplastic cell line (designated HuT11) has been established in continuous culture from an involved lymph node of a patient with Stage IIA Hodgkin's disease of the mixed cellularity type. The HuT11 line has been morphologically heterogeneous, consisting of mononucleate lymphoid-like cells, polygonal epithelioid cells, and mono-, bi-, and multinucleate giant cells. Four clones initiated from isolated binucleate giant cells of the HuT11 line also have been successfully established as continuous cell lines. The cloned lines have been morphologically distinct and more homogeneous, although typical giant cells have consistently appeared throughout the long-term culture of each. The HuT11 lines have grown as monolayers in McCoy's Medium 5A supplemented with 10% fetal calf serum, with generation times of 12 to 14 hr and high saturation densities. Cytogenetic studies showed that early and later passages of HuT11 cells were aneuploid, and all cell lines were successfully heterotransplanted in the hamster cheek pouch. Repeated indirect immunofluorescence examinations have shown each cell line to be negative for Epstein-Barr virus nuclear antigen. Indirect immunofluorescence tests in which monospecific immunoglobulins were used revealed positive membrane reactions for the gamma (heavy)-chain and kappa (light)-chain of human immunoglobulin G in approximately 20% of viable cells in each line; however, direct immunofluorescence with anti-human immunoglobulin G F(ab')2 reagent failed to confirm these reactions. Rosette tests for B- and T-lymphocyte and macrophage membrane receptors yielded negative results. All cell lines were strongly phagocytic for latex particles and neutral red dye. Cytochemical stains of the monolayers revealed abundant esterase, fluoride-resistant nonspecific esterase, acid phosphatase, and leucine aminopeptidase activities, while lysozyme assays were negative. Although some properties of the HuT11 lines have suggested a macrophage derivation, an undifferentiated lymphoid cell origin of the Hodgkin's neoplastic cell remains a possibility.