OBJECTIVE To develop a "miniorgan" culture of human endometrium dated according to the LH peak in order to study the process of implantation of a human blastocyst in vitro. METHODS Preliminary results of a more extensive study with the same objectives. METHODS Hospital-based unit of reproductive health and university-related reproductive research laboratories. METHODS Six apparently healthy women with normal regular menstrual cycles providing endometrial material 4, 5 and 6 days after the LH peak. METHODS One part of the biopsy was used for histologic dating. The other part was incubated in culture medium RPMI-1640 and exposed to the contact with one to three embryos fertilized in vitro. METHODS Biopsy material before and after incubation was studied by morphometric methods in a light microscope. The histologic changes were recorded by photomicrographs. RESULTS Implantation of an embryo obtained 4 days after IVF penetrated the lining endometrial epithelium of a biopsy obtained 4 days after the LH peak and kept in vitro for 24 hours. The embryo was placed on the lining epithelium of the endometrium within 3 hours after the biopsy was taken. CONCLUSIONS A new method to study the process of implantation of a human embryo in vitro has been described, allowing the embryo to penetrate the lining endometrial epithelium in a biopsy obtained at LH +4/+5.