Administration of a mixed copolymer of phenylmethylcyclosiloxane to rabbits on Days 4 and 5 of pregnancy prevented implantation. Cytochemical and ultrastructural studies revealed that the macromolecular material was taken up by 6-day-old trophoblasts (but not the embryonic cells) and was sequestrated in vacuoles. The presence of acid phosphatase in some of these vacuoles identified them as belonging to the lysosomal system. Uptake into lysosomes of macromolecular material brings about degenerative changes in the trophoblast, impeding successful implantation. No evidence was observed of the changes in uterine cells that normally accompany implantation, nor was any siloxane-like material seen in lysosomes in the uterine epithelial cells or macrophages.