Summary Staphylococcus antitoxin titers have been obtained from extracts of lymphocytes secured from a rapidly growing mouse lymphosarcoma. Comparison of these titers with those of the sera and normal lymphocytes of the same animals suggested that tumor tissue had a slightly higher antibody content. Tumor cells were capable of obtaining antibody from some other source in the body, presumably normal lymphocytes. The growth of an antibody-containing tumor transplant in normal mice was accompanied by the development of antibody-containing malignant cells. The normal lymphocytes of the host animal receiving this transplant contained antibody. There is a reversible exchange of antibody protein between normal and malignant lymphocytes.