Infant colic, a common disorder of infancy, is characterized by excessive crying and fussing. In this preliminary study we examined whether Neocate, an amino acid-based formula, would be accepted by formula-fed infants with colic, 3-7 wk of age, and whether Neocate would improve their symptoms. Six infants with colic were studied using Barr-type infant behavior diaries for 3-6 d on their current formula and then for 5-17 d on Neocate exclusively. All infants tolerated Neocate well and all improved, usually within 1-2 d. The total time spent crying and fussing was reduced by an average of 45%, representing a decrease of 1.0 to 5.2 h daily. After colic symptoms improved, infants were challenged with oral doses of 75 mg of bovine IgG at a 1 mg/ml concentration in order to assess its potential role in colic. Bovine IgG challenges resulted in increased crying and fussing behavior, suggesting that this protein may be etiologically important.