Cryostat sections of epididymides from mice were stained with monoclonal antibodies against immuno-competent cells. This investigation was undertaken to gain basic data about the distribution of macrophages. T lymphocytes, MHC class II and MIF in the normal murine epididymis to establish the mouse as a model for immunological epididymal research. The most important findings were as follows. (1) Macrophages, T lymphocytes, MHC class II and MIF positive cells were distributed similarly in the caput, corpus and cauda epididymis. (2) Macrophages were the most frequent leucocytes and the majority were located in the peritubular layer. (3) The MHC class II determinant was also expressed mostly in the peritubular layer and interstitium. These cells were similar in appearance and location to macrophages. (4) Significantly fewer T lymphocytes were found and their main location was the interstitium. T-helper and T-suppressor/cytotoxic lymphocytes did not differ significantly in their regional or histological distribution patterns. (5) The ratio of T-helper to T-suppressor/cytotoxic lymphocytes was 1:1. (6) MIF was detected almost exclusively in blood vessels and the surrounding connective tissue. (7) No invasion of leucocytes into the epididymal lumen was observed. It is concluded that macrophages seem to be the most important immunological cell type in the murine epididymis.