The effect of the antisecretory factor (ASF) on experimental porcine enterotoxin-induced jejunal secretion was tested. The heat-labile enterotoxin (LT) from Escherichia coli and cholera toxin (CT) was used for challenge in ligated intestinal loops. Less than 10 units of ASF inhibited the LT-induced secretion, while that due to CT required more than 10 units of ASF. ASF was effective only when administered prior to toxin challenge, and could be given either intravenously or intra-intestinally. Mixing of ASF with specific anti-ASF antibodies prior to injection abolished its antisecretory effect. LT- and CT-induced secreted fluid contained equal concentrations of Na+, K+ and Cl-, and the ionic concentration was not affected by ASF. Less than 0.1 units of ASF per pituitary gland was present in 3- and 5-week old pigs, while it increased to 4.5 units in 28-week old animals, and to 12.2 units in pigs older than two years. However, after intra-intestinal vaccination with 2.0 mg CT, the pituitary ASF content in the 5-week old animals increased to 2.0 units within 24 h.