The phosphate-binding capacities of 19 liquid and solid aluminum hydroxide gel antacids were determined in vitro under varying pH conditions. The resulting data provide a basis explaining the phosphate-binding characteristics observed when patients are treated with long-term aluminum hydroxide therapy. No antacid, liquid or solid, showed significant binding at pH 1.0. Maximum phosphate binding (expressed as phosphorus; P) was observed at pH 2.0 and 3.0 for most antacids and decreased markedly at alkaline pH. The liquid antacids showed a significantly greater phosphate-binding capacity than did tablets or capsules (p less than 0.01). At pH 2.0, the liquid antacids bound a mean of 22.3 mg P/5 ml. At pH 8.0 binding was reduced to a mean of 7.3 mg P/5 ml. Significant interbrand differences were observed. At pH 2.0, the solid antacids bound a mean of 15.3 mg P/tablet or capsule. At pH 8.0, binding was reduced to a mean of 5.8 mg P/tablet or capsule. Interbrand differences, while substantial, were less than those observed among the liquid antacids. Variations in sodium and potassium content were clinically insignificant for most of the antacids in this study, while the differences in phosphate-binding properties were sufficient to warrant attention in the patient with renal failure.