Human transfer factor was prepared from "buffy coat" of human donors by ultra filtration. These batches of transfer factor were tested for their ability to increase DNA-synthesis of antigen(mitogen) incubated lymphocytes. An increase of DNA-synthesis was measured by the ratio of appr. 2. Column chromatographic separation of transfer factor on sephadex G-10 or DEAE cellulose columns showed an elution pattern with many peaks. In the lymphoblastogenese assay some peaks were found to be stimulatory, others in contrast inhibitory. It seems likely from our results that transfer factor represents a mixture of different chemical compounds with activating or inhibitory action on lymphoblastogenesis.