The present study was carried out to investigate the influence of inhibitor factors on the immunological response of the mother. Using a FIVET program (in vitro fertilization and embryo-transfer) we were able to detect in vitro the modulation of lymphocyte response by hormones, utilizing subjects receiving hormone therapy. A new assay has been used to study the antibody response. The lymphocytes were adhered during the culture to the plastic surface of microplates, and the modulation of the expression of the surface IgG was detected with an E.L.I.S.A. assay in the same culture plate. Many hormones were tested in vitro using this assay. The addition to the lymphocyte cultures of hCG and oestradiol give a variable effect. This effect is concentration dependent and whilst in all instances the higher dosages were inhibitory, in same subjects the lower dosages had a stimulatory effect, but if the subjects were treated in vivo with hCG, then an inhibitory effect was observed in vitro even using very low concentrations of hormones. This could suggest that these hormones have an inhibitor effect on the lymphocyte response, inhibition that was enhanced when the lymphocytes were "primed" in vivo.