ABSTRACT: In previous studies, we reported that the injection of monoclonal antibody 14–30, specific for a T-cell suppressor factor (TSF), into mice during early stages of pregnancy could decrease the percentage of females that maintained pregnancy. In addition, further work has demonstrated the presence of an immunoreactive protein in fetal and maternal tissues with physiochemical properties similar to TSF. However, one alternate explanation for the antipregnancy effects of the injections of monoclonal antibody, not related to a specific role for TSF in early pregnancy, is the possibility of direct effects upon the embryo or embryonic antigens that prevent continued embryonic development. In the present studies, early preimplantation embryos were incubated with the monoclonal antibody 14–30 specific for TSF and the subsequent development of the embryos examined. The results of these studies demonstrate that monoclonal antibody, which has been shown to bind T-cell suppressor factor and has antipregnancy effects when injected in vivo, does not interfere with development of preimplantation or implantation stage mouse embryos in vitro.