Protocols are described for the isolation from human placental extracts of a preparation which is active in the rosette inhibition assay by inducing an increased rosette inhibition titre. In this respect the preparation mimics the effects of pregnancy sera, an ability ascribed to the presence in these sera of a so-called 'early pregnancy factor' (EPF). In addition to this activity in the rosette inhibition assay, the preparation was also shown to modulate the expression of cell surface immunoglobulin on peripheral blood mononuclear cells. The polypeptide composition of the preparation was relatively simple, as revealed by SDS-PAGE. There was a major 12 kd polypeptide previously isolated in a rosette inhibition titre-active preparation from ovine placental extracts, and a small amount of 68 kd polypeptide. N-Terminal amino acid sequence analyses obtained after blotting onto polyvinylidene difluoride membranes identified the latter polypeptide as serum albumin and the major 12 kd polypeptide as human thioredoxin. Gel permeation analysis partially resolved activity expression away from the 12 kd polypeptide as activity expression was also found to be associated with low mol. wt material. It is concluded that EPF activity expression in pregnancy sera involves molecules related to and associated with thioredoxin.