Co-administration of crude or water-soluble extracts from syngeneic placentae with allogeneic tumor cells markedly improved the tumor allograft survival. The activity was present in a fraction of MW < 60 000 daltons, suggesting that it was not an immunoglobulin. Pre-injection of the extract with allogeneic spleen cells prevented or retarded a second set rejection reaction. This effect was shown to be due to the generation of suppressor cells. It is suggested that placental products, in conjunction with alloantigens, induce a specific ‘immunodeviation’ or ‘split tolerance’ state characteristic of pregnancy.