A non specific immunosupressive factor able to block an in vitro cytotoxicity reaction is demonstrated in the serum of pregnant women. A solution containing this blocking factor is obtained by gel filtration and precipitation of plasma by polyethylene glycol 4 000. Then after immunisation of rabbits the immune serum can be used for affinity chromatography. An alpha 2 glycoprotein has been separated which inhibits the in vitro cytotoxicity reaction and whose molecular weight determined by gel filtration and polyacrylamide gel is about 200 000 daltons.