The random faecal alpha-1-antitrypsin (AT) excretion (mg/g dry weight of stool) was measured in 30 infants and children (mean age 10.8 +/- 8 mo.) with protracted diarrhoea (duration greater than or equal to 21 days) and failure to thrive and 27 normally nourished children (mean age 13 +/- 4.5 mo.) without any gastrointestinal symptoms in the preceding 12 weeks. The associated factors in patients with protracted diarrhoea and their mean faecal AT during active disease and 3-4 weeks after recovery were as follows: Enteropathogenic E. coli 5 (7.9 +/- 5.5; 3.2 +/- 0.6), Giardia lamblia 4 (3.9 +/- 1.8; 2.5 +/- 0.7), Salmonella typhimurium 3 (4.0 +/- 0.2; 3.8 +/- 0), secondary carbohydrate intolerance 11 (2.5 +/- 0.9; 2.4 +/- 0.8), and others 7 (3.4 +/- 0.7; 3.0 +/- 0.5), respectively. Of all the patients with protracted diarrhoea the mean AT in the E. coli, Giardia and Salmonella groups were significantly higher than the mean in the control group (2.1 +/- 0.8) and following treatment and recovery the values were comparable to that in the controls. All the 6 patients with very high faecal AT (greater than mean + 3 SD of controls) were associated with an enteric pathogen.