Administration of cholera toxin to an animal can protect it against subsequent administration of the toxin or viable Vibrio cholerae. This protection is usually explained in terms of neutralisation of the toxin by secretory IgA before it penetrates the intestinal mucosa. In 1984 Lange and Lönnroth1 reported that extracts from hypophysis, brain and jejunal mucosa, obtained from rats immunised with cholera toxin administered intravenously or orally, attenuated fluid secretion induced by cholera toxin in jejunal loops. This effect was not seen with extracts from other organs (pancreas, spleen, adrenal glands) or from control rats. The antisecretory activity of the pituitary material, known as antisecretory factor (ASF), increased as the number of immunisations increased. ASF was found to be a protein with a molecular mass considerably lower than that of IgA. This observation and the origin of ASF suggested that it is not secretory IgA. Subsequently, Lange and Lönnroth2reported that ASF was also found in bile and milk from rats immunised with cholera toxin. Further studies demonstrated antisecretory activity in the pituitary in rats, pigs and humans who had not been immunised against cholera toxin.3

Conventional biochemical separation techniques were used initially to try to isolate and characterise ASF.3 The ASF bioassay comprises inhibition of cholera toxin induced fluid secretion in rat jejunal loops. The apparent molecular mass of ASF varied widely from …