Mouse immunoglobulin G (IgG) was iodinated with 125iodine (I) and injected intravenously into pregnant mice in order to examine whether mouse granulated metrial gland (GMG) cells are able to take up IgG in vivo. The mice were injected intravenously on days 8, 12 or 16 of pregnancy and killed either 5 min, 2 h or 24 h after injection. Implantation sites and spleen, thymus, liver and para-aortic lymph nodes were fixed and autoradiographs of sectioned (1 micron) material prepared to examine the distribution of labelled IgG. In general, at all stages of pregnancy and time intervals examined after injection of the 125I IgG, radioactivity was detected at higher levels in blood vessels than in tissue spaces of the same regions. No evidence for the uptake of radioactive IgG by normal GMG cells in the decidua basalis, metrial gland or in the maternal blood spaces of the labyrinthine placenta was found. The only GMG cells which had accumulations of silver grains showed signs of pyknosis. The uptake of IgG by stromal cells in close proximity to GMG cells and the distribution of radioactivity in the extravascular tissues showed that the intravenously injected 125I IgG was available to the GMG cells. Accumulations of silver grains were a prominent feature of the regions immediately adjacent to most GMG cells in the placental labyrinth and some were clearly associated with degenerate layer 1 trophoblast cells. The radioactivity detected in degenerate GMG cells and degenerate layer 1 trophoblast cells may be the result of nonspecific uptake as a consequence of the cells' death.