Amniotic fluid was found to cause significant leukocyte migration enhancement during the second and third trimester of pregnancy and in the early postpartum period when compared to the migration area obtained with an ovarian tumor homogenate antigen (p < 0.01), choriocarcinoma spent medium (p < 0.01), and placental pool homogenate (p < 0.01). Only borderline significance (p < 0.1) was obtained when migration enhancement with AF was compared between pregnant and nonpregnant female control patients, indicating minimal unspecific activity of AF. Migration enhancement with autologous amniotic fluid was slightly larger than with homologous amniotic fluid, but the difference did not reach significance (p < 0.4). None of the control antigens caused migration enhancement; placental pool homogenate in concentrations above 4 mg. per cent caused migration inhibition but did not in lower concentrations. The enhancing effect of AF could be abolished by dilution but not by addition of excessive antibody to estrogen or HCG. It is suggested that a blocking factor is present in AF preventing recognition of fetoplacental antigen by the maternal immune system. Thus in vitro leukocyte migration enhancement may correlate to in vivo graft enhancement.