Antibody‐dependent cellular cytotoxic (ADCC) activity of cells in rat metrial glands at days 13 and 20 of pregnancy has been examined. A51chromium‐release cytotoxicity assay was carried out using Yac‐1 cells and P815 cells as targets in the presence of the corresponding, heat inactivated rat antibody or non‐immune rat serum. No killing was observed when effector metrial gland cells and target cells were cultured in the presence of non‐immune rat serum. In the presence of rat antibody toYac‐1 cells, or rat antibody to P815 cells, cells from the metrial glands of rats killed at day 13 of pregnancy demonstrated ADCC activity, but cells from the metrial glands of rats killed at day 20 of pregnancy showed no ADCC activity. Spleen andperitoneal exudate cells displayed ADCC activity. Immunohistological and immunocytological studies showed OX‐6, OX‐34, OX‐35 and OX‐42 positive cells were present at days 13 and 20 of pregnancy, in sections of metrial glands and in the cell suspensionsprepared for the cytotoxicity assays. It is suggested that macrophages are the cells most likely to be responsible for the ADCC activity in th