A completely automated method for the simultaneous or individual determination of calcium and phosphorus in serum and urine without preliminary sample treatment is reported. Separation of calcium and phosphorus from interfering materials is achieved by dialysis under acid conditions. The dialyzed calcium is determined colorimetrically using a metal complexing dye (cresolphthalein complexone) in an alkaline medium. Magnesium, phosphate, citrate, sulfate, etc., did not introduce significant errors under the test conditions. Inorganic phosphorus is determined by a modified Fiske-SubbaRow procedure. Reproducibility and recovery data as well as comparisons with manual methods are presented.