Summary: In a previous publication (5) we indicated that mature and immature lymphocytes and polymorphonuclear leukocytes possess specific antigenic substances which can be recognized by agglutination with specific antisera. It was also shown that there was no cell specificity for leukocytes obtained from normal and leukemic people. In the present article, it is demonstrated that cells of leukosarcoma and lymphosarcoma (of the reticulum cell type and of malignant lymphocytoma, Ewing) are antigenically alike but are dissimilar from the cells of acute and chronic lymphoid leukemia. These observations suggest a difference in chemical structure of the two types of cells, leukemic and lymphosarcomatous, the latter representing a malignant neoplasm.