The determination of calcium in erythrocytes by the method PIXE requires the preliminary elimination of large amounts of potassium. The best results, with regard to potassium removal and calcium yield, were obtained by carrying out a coprecipitation of calcium and strontium in 80% nitric acid. The development of the method has shown the importance of the total elimination of leucocytes and platelets from the serum in order to avoid erroneously high values for erythrocyte calcium.