A practical procedure is described for the analysis of ultramicro quantities of serum for calcium and potassium using the x-ray spectrometer. The serum is applied and dried in a confined spot on filter paper. The sample is then exposed to the x-ray field. The Kα lines of these elements are isolated and their intensity measured, with a flow proportional counter in a helium atmosphere. Reproducibility is of the order of ±5% (2σ) for these elements, with an approximately one-minute counting time. The results compare favorably with those obtained with the flame photometer for potassium and the Clark-Collip method for calcium. The x-ray method for calcium is more rapid and simpler than methods generally used in the clinical laboratory. The flame photometer is faster at present, but the instrument is readily automated by placing the serum sample in confined spots on tape. In this form it will probably find routine use for the electrolytes and other elements in the near future.