Lithium has been used as a medicine for the remedy and prevention of manic-depressive psychosis. It is, however, essential to monitor the blood lithium concentration of patients during their hospital-treatments, because the intake amount of lithium as a remedy is reflected in the blood lithium concentration, which is significantly close to a toxic level. Since continual lithium intake with daily meals is expected to prevent manic-depressive psychosis at an early stage, it is important to know the food lithium content. In this work, the effects of matrix constituents (Na⁺, K⁺, Ca²⁺, Mg²⁺ and PO₄³⁻) of food on the determination of lithium by flame photometry were studied. Since there was no interference of the matrix ions, the method was applied to food samples: fresh fish (30∼84 ng/g), dry-processed fish (731 and 1790 ng/g), table-salts (667∼2340 ng/g), infant formulas (120∼160 ng/g) and eastern medicines, such as mammalian bone and oyster-shell (18700 and 20200 ng/g).