Abstract A sensitive and accurate method for measuring calcium and magnesium in serum and urine by atomic absorption spectrophotometry is described. Phosphate depression has been eliminated by the addition of strontium, and 0.1 N perchloric acid has been used to remove protein interference. Accuracy for both metals has been established by comparison with a dry ashing technique, and for calcium by comparison with an EDTA titration using a calcein end point. A technical error of ± 0.13 mg/100 ml for calcium and ± 0.04 mg/100 ml for magnesium has been obtained in blind duplicates. The normal range found for calcium is 9.3–10.3 mg/100 ml and 1.9–2.3 mg/100 ml for magnesium. Possible interference from urinary constituents, flame conditions, the use of a variable atomiser, effect of dilution and other parameters have been investigated and do not affect the results for either element.