To avoid the use of aluminium as a phosphate binder, patients on CAPD who were stable were dialysed against a peritoneal dialysis fluid which was magnesium free. A mixture of calcium and magnesium carbonate was used as a phosphate binder over a period in excess of 1 year. Vitamin D analogues were used in the majority. Results show satisfactory control of hyperparathyroidism with mean parathyroid hormone concentration for the group of 121 pg/ml (normal < 100 pg/ml), calcium concentration of 2.41 mmol/l, magnesium 0.97 mmol/l, phosphate 1.36 mmol/l and aluminium 0.35 mmol/l (normal < 0.2 μmol/l). These results were as good as and better in some respects than a minority using calcium carbonate alone or remaining on aluminium hydroxide, the latter remaining on Mg-containing CAPD fluid.