Two in vitro cell lines (L428, L439) were established from pleural effusions of two patients with Hodgkin's disease. The histological diagnosis was ascertained in both cases by two independent pathologists. The neoplastic nature of the culture cells is indicated by the demonstration of several structural and numerical chromosome abnormalities associated with a monoclonal pattern of marker chromosomes. Heterotransplantation in nude mice was achieved by intracranial inoculation and by subcutaneous transplantation of cultured cells embedded in a plasma clot. EBV-specific antigens (EBNA, VCA) were not detectable in either cell line. Ia-like antigens, receptors for T cells, acid phosphatase and acid esterase were shown to be present in the cultured cells. The L428 and L439 cell line lacked surface- or cytoplasmic Ig, HTLA, receptors for C3b, C3d, IgG-Fc, mouse E or sheep E, and were devoid of lysozyme, peroxidase and chloracetate esterase. These features do not correspond to those of B cells, T cells, myeloid cells, monocytes or macrophages; the morphology and the marker pattern of the culture cells, however, is identical with that of freshly obtained Hodgkin (H)- and Sternberg-Reed (SR)- cells, except for the lack of Clg in the in vitro cells, which is explained by the culture conditions. These findings suggest that the L428 and L439 cell lines are indeed derived from H- and SR-cells and offer the possibility of gaining new information upon the nature of Hodgkin's disease.