The lack of sensitive and relatively non-invasive measures has hampered monitoring the clinical course of spontaneously developing colitis in IL-2-deficient (^−/−) mice. We selected (i) to study the correlation of the acute phase plasma proteins serum amyloid A (SAA) and serum amyloid P component (SAP) levels with colonic disease and (ii) to characterize the amyloidosis in the IL-2^−/−animals. IL-2^−/−mice exhibited increasing severity of gross intestinal inflammation with age, confined to the distal colon. Histologically, the colonic disease score increased serially in IL-2^−/−animals. Wild-type mice showed no activity, while 16-week-old IL-2^+/−animals had minimal colitis with small ulcers and lamina propria inflammatory infiltrate. Periportal hepatitis was present and positive Congo red staining indicated amyloidosis of the liver and spleen in 16 week IL-2^−/−mice. SAA immunostaining in the liver and spleen was increased in the 8 week and 16 week IL-2^−/−and 16 week IL-2^+/−animals indicating AA amyloid deposits. Plasma SAA and SAP levels were markedly elevated, and generally preceded the onset of colitis and reflected its severity. Northern analysis showed markedly increased SAA expression in the liver and intestine of IL-2^−/−and intestine of IL-2^+/−16-week-old animals. Increased intestinal expression of SAA3 (lamina propria macrophages) indicates local inflammation in IL-2^+/−animals at 16 weeks. Treatment of 3-week-old animals with systemic IL-2 or IL-1 receptor antagonist (IL-1ra) delayed inflammation, postponed the increase in SAA levels and minimized disease onset. These results further demonstrate that IL-2 plays a significant role in normal immune responses in the body and that plasma SAA levels both reflect colonic disease severity and may indicate subclinical disease in both IL-2^−/−and IL-2^+/−mice. Furthermore. The mechanism of IL-2-deficient induced colitis appears to be mediated in part through the increase in IL-1. In addition, the IL-2^−/−mouse of spontaneous enterocolitis may provide a unique system for studying spontaneously developing AA amyloidosis.