Detection and separation of two serum factors responsible for depression of lymphocyte activity in pregnancy

Studies using the rosette inhibition test have demonstrated a depression of lymphocyte activity in pregnant mice which occurs as early as 6 hr after fertilization and persists until just before delivery. This depression of lymphocyte activity is induced by two serum factors, the relative contributions of which vary with the time of gestation. The first, a high molecular weight early pregnancy factor, appears within hours of fertilization and then slowly declines in activity as gestation proceeds. The second, a lower molecular weight substance which cross-reacts with antiserum to human chorionic gonadotrophin, appears after the formation of the trophoblast at 4--5 days and persists until parturition.