Almagate (Al2Mg6(OH)14(CO3)2 X 4 H2O, Almax), a crystalline hydrated aluminium-magnesium hydroxycarbonate, was compared with a number of other known antacids in a battery of tests designed to demonstrate the properties of an ideal antacid. In dynamic tests for measurement of velocity of neutralization, total acid consuming capacity and duration of activity, almagate like other crystalline aluminium magnesium hydroxide derivatives such as hydrotalcite and magaldrate was rapidly acting, neutralized a high proportion of its theoretical acid consumption and maintained gastric pH between 3 and 5 for a prolonged period of time. It was more active than these two substances in respect of rapidity of action and did not lose antacid activity as the pH was raised. Like these compounds it had considerable advantages over the amorphous gels and co-gels of aluminium and magnesium hydroxides or hydroxycarbonates in respect of both rapidity of action and acid consuming capacity. It inhibited the activity of pepsin and was shown to adsorb bile acids and had a low sodium content.