Calcium in human and rabbit plasma was determined by absorption spectroscopy, using the calcium standard solution in which contains stronthium, hydrochloric acid, depicked plasma, and various serum constituents attempted by Sekiguchi et al. This method is very simple and satisfied in accuracy to avoid certain anionic and cationic interferences. The effects of 12 kinds of interferences, sodium, potassium, and magnesium enhanced the calcium absorbance, while zinc, ferrous, cupric, phosphate, sulfate, bicarbonate, carbonate, EDTA, and oxalate depressed the calcium absorbance, especially in phosphate and oxalate.