Further studies on splenic material inhibiting the growth of lymphocytes in vitro

A material inhibiting growth of lymphocytes from different lymphoid organs and from different species was purified from calf spleen. A factor with a molecular weight of about 45,000 dal (estimated by gel chromatography) inhibited DNA synthesis and mitosis irreversibly in thymocytes and caused degenerative changes in the nucleus and the cytoplasm of the thymocytes as judged by electron microscopy. However, no decrease in cell number and no increase in dye uptake in a dye exclusion test were found. Growth inhibition was also demonstrated for human melanoma cells, but not for rat liver cells. The cytotoxicity and the lack of absolute lymphocyte specificity of the factor speak against its character as a lymphocyte chalone. A probably identical factor was purified from calf liver, but was not found in extracts of calf thymus.