An indirect immunoperoxidase technique was used to study the distribution of IgA, IgG, IgM and albumin in the uterus of primigravid mice. As pregnancy proceeds from days 2 to 6, IgA-containing plasma cells concentrate around the uterine glands; IgA is found in an increasing number of glands and then in the uterine lumen. At the same time the stroma is progressively invested by IgG, but IgG plasma cells are not present in significant numbers and IgG is very rarely found in glands. IgM remains in blood vessels until day 5 when it is present in small amounts in the stroma. Albumin tends to follow a pattern similar to that of IgG but in addition is present in the lumen and in a few cells in the luminal epithelium. The growing decidua does not contain Ig. As the embryo reaches the uterine lumen, IgA produced locally by plasma cells, evidently is secreted into the uterine lumen via uterine glands while IgG infiltrates the stroma as a result of increased permeability of the uterine capillaries at the time of implantation.