In the milky spots, precursors of cells belonging to the mononuclear phagocyte system (MPS), such as monoblasts, can be found on the basis of ultrastructural endogenous peroxidase cytochemistry. Therefore, in the present study, we investigated the milky spots using a panel of monoclonal antibodies, especially antibodies (ER-MP) that recognize macrophage precursor antigens. Early macrophage precursor antigens ER-MP12 and ER-MP58 were detected only on cells localized inside the milky spots. On the other hand, an antigen which disappears late in the course of macrophage differentiation, ER-MP20, was detected in high amounts on cells both inside and around the milky spots. This clearly indicates that macrophage precursors are centrally localized inside the milky spots, while more differentiated cells are found in peripheral areas. Moreover, long-term culture of milky spot tissue resulted in the forming of a monolayer of stromal cells which supported macrophage proliferation in vitro. In conclusion, both in situ and in vitro studies demonstrated that mouse milky spots have a microenvironment in which precursor cells of the MPS can home and proliferate, illustrating that milky spots play a role as a source of local macrophage generation, e.g. that of the free peritoneal macrophages.