The presence of large numbers of polymorphonuclear leucocytes in mixed leucocyte cultures of two unrelated donors results in higher levels of blastoid transformation than when polymorphs are absent. Cultures with large numbers of polymorphs also have many more macrophages present after 3 days in vitro than the cultures without polymorphs. It is suggested that polymorphs affect the blastoid transformation of small lymphocytes by causing some small lymphocytes to become macrophages during the first few days of culture. The macrophages then take up the foreign leucocyte antigens present and pass the antigenic information on to small lymphocytes which then undergo blastoid transformation.