Fecal alpha-1-antitrypsin in infants. Reference values and in acute diarrhoeal diseaseAlpha-l-antitrypsin (a-l-AT) was determined in faeces by agorose radial inmunodiffusion plates in order to establish reference values in random faecal samples, and to determine the magnitude of intestinal protein loss in acute diarrhoea and its eventual association with the aetiology and clinical progress of the disturbance. Fifty three patients 1 to 11 months of age were studied. Fifteen infants admitted with acute respiratory disease, were investigated when fully recovered, just before discharge: reference values found were (mg/g dry weight): 4.80 to 50.80 (x ± 2 SD n _i)-Thirty eight cases with acute diarrhoea were studied within 24 hours after admission: increased faecal a-l-AT loss was detected in 47.3% of these patients, with 3 cases showing loses greater than 100 mg/g. Severe malnutrition was not observed. Faecal. a-l-AT loss was neither related to nutritional status nor to the aetiology of diarrhoea. However, increased loss was associated to watery stools, prolonged hcapitalization and probably to protracted diarrhoea (p -0.05).