The platelet-derived factor (PDF) which has the suppressive effect against plasma blocking factor (PBF), was further characterized and partially purified. PDF was pH and heat stable, but it was rendered unstable by treatment with proteolytic enzymes. The biological activity of PDF was associated with four entities of molecular weights; 150, 000 (Fr.I), 130, 000 (Fr.II), 85, 000 (Fr.III) and 50, 000 (Fr.IV) determined by gel filtration in 1M acetic acid. The fraction IV showed the highest suppressive effect against PBF as well as the highest stimulative activity of DNA synthesis in bovine lymphocytes and murine or bovine fibroblasts.