Mg, K and Na were measured in erythrocytes, lymphocytes and serum obtained from 20 normal subjects. The cells of 20 ml of blood were separated in Ficoll gradient, washed with a choline buffer and digested with H2SO4. The cations were measured by atomic absorption photometry, protein was determined by the Lowry method and the results were expressed as nmol/mg of protein. Intracellular lymphocyte concentrations (mean +/- SD, n = 20) were: Mg: 57.6 +/- 5.1, Na: 30.0 +/- 7.9, K: 614.3 +/- 69. In the red cells, ion concentrations were: Mg: 7.97 +/- 0.8, Na: 20.9 +/- 4.8, K: 343.0 +/- 42.0. There was a highly significant correlation between Mg and K concentration in erythrocytes (p less than 0.001) and lymphocytes (p less than 0.001). A negative correlation (p less than 0.05) was also found between lymphocyte and erythrocyte K concentration and K serum concentration.