In the last 4 years we have established five long-term cultures from tumor material of Hodgkin's disease. The in vitro cells have malignant characteristics and represent the in vivo Hodgkin- and Sternberg-Reed-cells as shown by the identity of multiple properties. Common immunological, functional, and morphological assays did not characterize the in vitro cells as a known cell type of lymphoid, myeloid, or monocytoid tissue. The in vitro Hodgkin's disease cells are biologically active by producing factors involved in regulation and promotion of immunological response and granulopoiesis. The relevance of the findings for pathogenesis and clinical appearance of Hodgkin's disease is discussed.