A partly purified calf spleen extract which inhibits lymphocyte proliferation in a tissue-specific, species-nonspecific, nontoxic, and reversible way, and may thus contain a chalone-like activity, has been further characterized. Our results show that the inhibitor works in G1-phase. The activity is resistant to heat, trypsin- and ribonuclease-treatment. It appears to be an acidic molecule and it does not appear to contain ribonucleic acid. These properties are compared to the characteristics which have been described for tissue-specific inhibitors of lymphocyte proliferation by other authors.

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A partially purified calf spleen extract, which exhibits the properties of a lymphocyte chalone (tissue-specific, species non-specific, non-toxic and reversible acting, endogenous inhibitor of cell proliferation) as concerning biological activities, has been described earlier. This fraction has been further purified by ion exchange chromatography with DEAE-Sephacel to a 10,000-fold purification. The most active fraction inhibited 3H-thymidine incorporation into phytohemagglutinin- or pokeweed-mitogen-stimulated lymphocytes by 86% resp. 70%, when given in a concentration of 100 ng/ml. Ehrlich-ascites-cells were not inhibited by the same concentration. The inhibiting activity is compared to other published lymphocyte inhibitors.