Leukocyte migration enhancement (LME), a common denominator of pregnancy and malignancy, was investigated in order to determine the immunologic factors and processes involved in this in vitro assay. Reproducibility of results was confirmed by parallel investigations of single patients in an independent laboratory. Preliminary evidence also suggests that the phenomenon of LME is observable not only with the leukocyte migration assay, as performed in the present study, but also with the "indirect" migration inhibitory factor (MIF) assay. Highly significant correlation between results obtained with plasma (PL) and amniotic fluid (AF) was observed (p < 0.00089). No significant correlation between LME and IgG concentrations as well as complement (C3) concentrations in AF and PL could be detected. C3 concentrations correlated significantly to gravity (p < 0.007) and parity (p < 0.003). No immune complexes were detected in either AF or PL samples. An unspecific protein effect as the cause of LME was ruled out in showing no significant correlation between LME and protein concentrations of AF and PL. A highly significant correlation between antigen concentration in PL and migration area was demonstrated (p < 0.00001). It is suggested that leukocyte migration inhibition (LMI) represents a state of antigen excess, while LME represents antibody excess. Through selective removal of the IgG immunoglobulins LME could be converted into LMI, indicating an IgG subfraction as the responsible immunoglobulin in the phenomenon of LME.