Uptake of uteroglobin (UGL) by day-6 rabbit blastocysts and the intracellular fate of this protein were studied by light- and electron-microscopic autoradiography, immunocytochemistry and acid-phosphatase cytochemistry. UGL, labelled with N-succinimidyl-(2-3-3H)-propionate, was administered to embryos in vitro for 25 min to 4 h. The kinetics, determined from light-microscopic autoradiographs, showed a continuous uptake of the labeled protein over a 4-h period of incubation. At the ultrastructural level, increasing numbers of silver grains and an intense UGL immunoreaction in protein vacuoles and crystalloid bodies of trophoblast cells indicated that 3H-UGL had accumulated in these organelles. The presence of crystalloid inclusions in protein vacuoles suggests their origin by a condensation of the protein content, including UGL. Lysosomes containing radioactivity were rarely found, suggesting a very low degradation rate of the 3H-UGL. Protein vacuoles and crystalloid bodies exhibited no acid phosphatase reaction. The enzyme was mainly found outside the basal and lateral cell membranes of trophoblast cells, and on the rough endoplasmic reticulum of endoderm cells.