Lymphocytes were classified according to their content of mitochondria. It was shown that there is a size correspondence between lymphocytes in supra-vital preparations and supra-vitally stained lymphocytes in dried films. The mean diameter of the lymphocytes in all mitochondrial classes was diminished by spreading and drying on microslides but the decrease was less marked in the cells having the highest number of mitochondria (mitochondrial class VI). It was also shown that some cells of this class become larger upon being spread and dried. The largest lymphocytes in both supra-vital preparations and dried films of supra-vitally stained lymphocytes belong to mitochondrial class VI. Comparison of the size of lymphocytes in conventionally stained smears and in dried films of supra-vital preparations indicate that the largest lymphocytes in each type of preparation are the same cells. The largest lymphocytes in conventional smears must therefore belong to mitochondrial class VI, and according to earlier knowledge of the relation between cell aging and mitochondrial content, must be young cells. In the same way it was shown that the smallest lymphocytes in conventional smears have fewer mitochondria (mitochondrial classes I-IV) than the largest lymphocytes and hence are older cells. All mitochondrial classes are represented in lymphocytes of intermediate size.