The human Sezary T-cell leukaemia line, HUT.78, represents a population of activated T cells, i.e. they are HLA-DR+ and IL-2R+. We have analysed the capacity of HUT.78 cells (1) to stimulate HLA-DR-specific T-cell lines or clones and (2) to be induced to synthesize IL-2 by anti-HLA-DR monoclonal antibodies. The results of our experiments show that HLA-DR molecules on HUT.78 cells can stimulate at least one HLA-DR-specific T-cell clone and can act as transmembrane signal transmitters.