The functional capacity of biologically active, high-affinity interleukin-2 receptors (IL-2R) was studied by means of interleukin-2 (IL-2) stimulation of blood mononuclear cells (BMC) from 22 patients with inflammatory bowel disease (IBD) and 24 controls. The spontaneous, as well as the IL-2-induced, proliferative responses were significantly decreased in patients with active IBD, whereas the expressions of biologically inactive, low-affinity IL-2R (i.e. TAC antigen or CD25) were significantly increased in the same BMC cultures. In contrast, no significant differences were seen between patients and controls when BMC were stimulated with a nonspecific mitogen (phytohemagglutinin). The results suggest that a downregulation of IL-2 responsiveness may contribute to decreased BMC proliferation in vitro in active IBD.