A polarographic method of assay of zinc in blood is described. 0.2–0.5 ml of the sample are decomposed with nitric and perchloric acids ; zinc is extracted by dithizone, then re-extracted by a diluted nitric acid solution. The accuracy of the method is ± 5%. A systematic study of the different losses which occurred during the procedure was made with ⁶⁵Zn, and all sources of contamination were thoroughly examined.