Calprotectin, a prominent cytosol protein in neutrophil granulocytes, was present in increased concentrations in stools from 50 of 53 patients with colorectal cancer, 32 of 40 patients with colorectal polyps, and all of 18 patients with gastric cancer. After radical surgery, faecal calprotectin levels reverted to the normal range in all but one patient with colorectal cancer. Calprotectin determinations are simplified by the stability of this protein in stools. Reliable estimates can be obtained in samples of only 5 g. On the basis of data from the literature, the test for calprotectin seems better than that for occult blood for the detection of gastrointestinal neoplasms.