Interleukin-1, a cytokine produced by macrophages and other tissue cells, has a major role in inflammatory and immunological responses. Increased levels of IL-1 activity have been reported in experimental colitis and in patients with active Crohn's disease (CD) and ulcerative colitis (UC). IL-1 beta release from fresh and cultured colonic biopsies and IL-1 beta plasma concentrations was determined in 15 patients with active UC, 16 with UC in remission and 10 normal control subjects. Biopsies, taken at colonoscopy were weighed, washed in 1 ml of 0.9% sodium chloride solution and then cultured for 24 h in 10% fetal calf serum/RPMI. IL-1 beta activity was determined by ELISA KIT (Cystron Biotechnology) in plasma samples, washing solution and the incubation medium. Very low levels of IL-1 beta were detected only in 3 plasma samples, all from active patients. Significantly more IL-1 beta was released from fresh and cultured colonic mucosa obtained from patients with UC in remission compared to normal mucosa (p less than 0.01). Furthermore, specimens from active UC released significantly more IL-1 beta than those from patients in remission (p less than 0.01). In conclusion, IL-1 may play an important role in mediating the inflammatory response in UC.