A diminished tolerance to the normal gut bacterial flora has been suggested to be pathogenic in ulcerative colitis (UC) and the aim of this study was to evaluate the priming effect of selected bacterial wall products on UC neutrophil granulocytes. Neutrophils from 10 UC patients and 10 healthy controls were primed with bacterial lipoprotein (BLP) or lipopolysaccharide (LPS) and subsequently activated. Extracellular superoxide production was measured by the cytochrome c reduction assay. Priming neutrophils with BLP or LPS dose dependently increased the superoxide production in both UC and controls (P<0.01), and BLP was more potent than LPS (P<0.05). No differences were found between UC and controls. UC neutrophils do not seem to have an intrinsic abnormality with reduced tolerance to bacterial substances. However, bacterial wall products such as BLP modify neutrophil tissue-destruction mechanisms and might be pivotal for perpetuation of chronic colonic inflammation.