The serum concentrations of serum amyloid A protein (SAA), C-reactive protein (CRP), alpha 1-antichymotrypsin (alpha 1-ACT) and alpha 1-acid glycoprotein (alpha 1-AGP) have been measured in eighty-six patients with Crohn's disease, twenty-five patients with ulcerative colitis and twenty-two patients with the irritable bowel syndrome. In the Crohn's and ulcerative colitis group significant increases in concentration were observed in all four proteins, which parallelled disease severity as defined by other conventional laboratory parameters formulated into a simple activity index. In the irritable bowel group no significant changes were seen. Serum amyloid A and CRP concentrations were significantly lower in ulcerative colitis than in Crohn's disease when mild, but did not differ significantly when severe. Serum amyloid A correlated well with CRP (r = 0.83) and alpha 1-ACT (r = 0.80), but less well with alpha 1-AGP (r = 0.65). Serum amyloid A was the most sensitive protein (77%) but had the lowest specificity (74%). C-reactive protein was less sensitive (58%) than SAA but had greater specificity (100%). Alpha 1-ACT had a sensitivity and specificity similar to CRP and, therefore, provided little or no additional information. Alpha 1-AGP, although also 100% specific, had the lowest sensitivity (34%) and, therefore, is probably the least useful acute phase monitor of inflammatory bowel disease. The role, and associated problems, of SAA measurements are discussed.