Acute phase proteins are released from the liver in response to cytokines, and measurement of serum concentrations offers a valuable means of assessing inflammatory bowel disease. C-reactive protein (CRP) is a participating prominent component of the acute phase response in active Crohn's disease. This study aimed at determining the comparative role of the cytokines interleukin-1 beta (IL-1 beta) and interleukin-6 (IL-6), in driving CRP production in inflammatory bowel disease, and to test the hypothesis that there is a difference in the profile of cytokines generated in these two conditions. Serum CRP, the release of the cytokines IL-1 beta and IL-6 from monocytes, and the ability of monocyte conditioned medium to stimulate CRP synthesis by hepatocytes in an in vitro system was measured in patients with ulcerative colitis and Crohn's disease. Monocytes from patients with Crohn's disease produced more 1L beta-1 than monocytes from patients with ulcerative colitis or normal controls. There was no increased tendency for monocytes from Crohn's disease patients to produce more 1L-6, so the greater circulating values of IL-6 reported by a number of authors in Crohn's disease may reflect the participation of a larger number of cells of the monocyte-macrophage series, or production of IL-6 by other cell types. Correlation of cytokine production by monocytes with in vitro CRP release from cultured hepatocytes in response to monocyte conditioned medium showed that, in that system, IL-1 beta was the stronger stimulus to CRP production. Some of the differences in the inflammatory processes of ulcerative colitis and Crohn's disease may reflect differences in the amount of IL-1beta and IL-6 generated from macrophages and monocytes.