[Attempts to characterize a tumor specific inhibitor extracted from the maternal part of the bovine placenta].

A fraction was isolated from the maternal part of bovine placenta, which significantly inhibits the incorporation of thymidine into the DNA of tumour cells. This factor has, however, only a limited effect on the same reaction in bone marrow cells or in fibroblasts. It is suggested that the factor enters the cell by an active transport mechanism and that the active part thereof is a protein. The protein pattern of the chromatin is not influenced by the factor as is the phosphorylation or acetylation of nuclear proteins.