We studied the effect of pregnancy plasma and monoclonal antibodies to the histocompatibility antigens HLA and DR on migration of normal leukocytes from capillaries. It was observed that plasma from women in the first trimester of pregnancy significantly enhanced leukocyte migration at a concentration of 10% (mean area of migration = 50.7 +/- 9.2 cm2), as compared with plasma from nonpregnant women (22.8 +/- 7.0 cm2; p less than 0.05). This effect was less during the second trimester (39.5 +/- 3.5 cm2; p less than 0.05) and no enhancement was noted with plasma obtained from the third trimester of pregnancy. Similar results were obtained with a 20% concentration of plasma also. On the other hand, the monoclonal anti-HLA and anti-DR antibodies had an inhibitory effect on migration of leukocytes. These results are discussed in relation to the immunoregulatory role of pregnancy plasma in the nonrejection of the fetal allograft.