IL-2 is a major growth factor for all T-cell subsets acting via a specific membrane receptor. To investigate its role in the pathogenesis of cutaneous T-cell lymphomas (CTCLs), we analysed the expression of high-affinity IL-2 receptors (IL-2Rs) on proliferating cells in these disorders. We showed by immunohistochemical double staining that most cycling cells do not express high-affinity IL-2Rs. Four T-cell lines were established from patients with CTCLs. Two lines required both IL-2 and IL-4 for growth, one line required IL-2 and one line IL-4 alone. The last of these lacked expression of the IL-2R alpha-chain. Thus, IL-2 may not be the only, or the most important, growth stimulus in CTCLs and reactive skin infiltrates. T helper cells, which dominated the infiltrate, might represent TH2 cells.