Serum lysozyme (muramidase) concentrations were determined in 55 patients with inflammatory bowel disease, 6 with miscellaneous bowel disease, 40 with pulmonary tuberculosis, and in 20 normal subjects. The mean (+/- SE) lysozyme concentration for each group was as follows: controls 6,95 +/- 0,36 microgram/ml; ulcerative colitis 9,61 +/- 1,02 microgram/ml; inactive Crohn's disease 7,61 +/- 0,53 microgram/ml; active Crohn's disease 20,77 +/- 2,17 microgram/ml; sputum-negative tuberculosis 13,05 +/- 1,06 microgram/ml; and sputum-positive tuberculosis 20,35 +/- 2,08 microgram/ml. The mean enzyme levels were significantly higher in patients with Crohn's disease than in those with ulcerative colitis (P less than 0,05) or in normal controls (P less than 0,01). Our findings suggest that serum lysozyme levels may be useful in differentiating active Crohn's disease from ulcerative colitis, but the results overlap somewhat. However, the enzyme level may be a useful index of disease activity in following up patients with Crohn's disease. As tuberculosis is endemic in this country it must first be excluded, because patients with pulmonary tuberculosis have similarly high levels of serum lysozyme.