During the process of follicular atresia, cells are observed to invade the zona pellucida (invasive cells) where they presumably play an important role in eliminating degraded oocytes. Although our preliminary studies have suggested that these cells may originate from granulosa cells and not from macrophages, a detailed morphological analysis of the cells has not been conducted. The objective of this study was to characterize the cells more precisely by electron microscopy and immunohistochemistry, using sexually immature mice. The results show that the invasive cells were first observed within advanced primary (non-antral) atretic follicles. The cells frequently contained cytoplasmic lysosome-like granules after passing through the zona pellucida. F4/80 and Mac-1, reported as macrophage-specific antibodies, were reactive with the cells in most cases, but some immunonegative invasive cells were also observed. The ultrastructural features of the invasive cells were quite similar to those of granulosa cells, not macrophages. Gap junctions, which are typical cytoplasmic structures of epithelial cells, were frequently identified between neighbouring cells. Although direct evidence indicating a contribution by the cells to the elimination of degenerated oocytes was not obtained, our results strongly suggest that the invasive cells originated from granulosa cells surrounding the zona pellucida, and that they may have a macrophage-like cell function for the elimination of oocytes from atretic follicles in mice.