The nature of Hodgkin and Reed-Sternberg (H-RS) cells and their normal counterpart remains a matter of controversy. Our recent investigations have suggested that H-RS-cells derive from certain activated lymphocytes of either B or T cell origin. In keeping with this concept, we were able to demonstrate that in the majority of cases of Hodgkin's disease the interleukin-2 receptor (IL2-R) was detectable on H-RS-cells by three monoclonal antibodies (anti-Tac, Tü69 and ACT-1) with the sensitive alkaline phosphatase anti-alkaline phosphatase (APAAP) tissue staining procedure. In extension of these studies, we precipitated IL2-R antigen from two established permanent cell lines (L540 and L591) derived from patients with Hodgkin's disease. Although these cell lines are different in nature, in that the L540 line shows rearrangement for the T cell receptor beta chain gene and the L591 line displays rearrangement for immunoglobulin genes (gamma heavy and lambda light chain), the antigen precipitated by monoclonal antibodies against IL2-R exhibited a molecular weight of approximately 53 kd on the L591 line and 58 kd on the L540 line. Thus, these data strongly support the view that the labelling of H-RS-cells in tissue sections for IL2-R was not merely due to cross-reactivities of the two anti-IL2-R antibodies with a determinant of an unrelated structure but represents true IL2-R molecules.