Freshly isolated fetal human colon cells were labeled with 32P-orthophosphate or 14C-amino acids and exposed to white blood cells from children with ulcerative colitis or from healthy controls. Exposure of the colon cells to patients' white cells led to a rapid isotope release, significantly higher than that obtained with normal white cells. After 150 minutes of incubation, 75 per cent of the total isotope present was found in the media of the colitis samples but only 40 per cent in those of the controls. Consistent results were obtained with white blood cells from 14 patients and 18 healthy individuals. Similar results were obtained with either fresh white cells or with white cells aged for 12 to 18 hours and consisting to 60 to 70 per cent of lymphocytes and to 20 to 30 per cent of large mononuclear cells.

No specific cytotoxic activity could be conferred onto normal white cells by pretreating them with patients' serum containing antibodies against colon antigen. The cytotoxic action of the patients' white cells was immunologically specific, since no difference from the controls was found in the isotope release when cells from other organs or animals were similarly treated. Preliminary experiments suggested that the patients' white cells could be desensitized by pretreating them with colon extract. For obtaining a significant cytotoxic effect of the patients' white cells, the presence of 10 to 20 per cent of fresh guinea pig or human serum in the incubation medium was required.

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