The barrier properties of an alginate antireflux agent (Liquid Gaviscon®, Reckitt and Colman, UK) were investigated using a portable cadmium telluride detector worn on the chest wall over the oesophagus to detect the reflux of radiolabelled food, with simultaneous oesophageal pH monitoring. A crossover study was performed in 12 healthy nonpatient volunteers.An oesophageal pH probe was placed coincident with a small &ggr;-detector strapped to the chest wall and the positioning verified using &ggr;-scintigraphy. A test meal designed to promote reflux was radiolabelled and administered to the subjects. On 1 occasion, subjects received only the meal, and on the second occasion the subjects also received a 20ml dose of alginate antireflux agent (Liquid Gaviscon®) 30 minutes after the meal had been consumed. The reflux of radiolabel and acid was monitored for 3 hours post-prandially.The antireflux agent was found to significantly reduce the amount of both acid and food reflux. The length of time for which the pH in the oesophagus was below 4 was 19.9 ± 1.6 minutes (mean ± SEM) in the untreated group, but 3.0 ± 4.0 minutes for the treated group. Although the absolute amounts of acid refluxed cannot be measured, it is possible to estimate the ratio of amounts of acid refluxed in treated and untreated groups. This ratio was 0.119 ± 0.028, indicating that the antireflux agent prevented the reflux of 88% of the acid refluxed by the untreated group. The ratio of the amount of food reaching the oesophagus in treated and untreated groups could be similarly estimated from the refluxed activity data as 0.095 ± 0.024, indicating suppression of food reflux by 90%. This study demonstrates that Liquid Gaviscon® significantly reduces the reflux of both food and acid.