Using an immunofluorescence technique on ovarian sections, zona-immunoreactive components were detected in the cytoplasm of the oocyte from the beginning of its growth, when it is surrounded by only a thin squamous follicular cell layer, up to the end of its growth. In parallel with oocyte growth, the staining intensity decreased in the ooplasm. No staining was observed in the cytoplasm of the granulosa cells during normal follicular development in adult cyclic females. However, staining of the granulosa cells was observed at some stages of follicular development in immature females. This staining was especially evident in the ovaries of immature females (22 or 26 days old) stimulated with PMSG. In addition, the staining of the granulosa cells was consistently observed in ovaries showing an abnormal histology. Increased staining of the zona at its outer and inner regions could be distinguished in normal follicles, but when staining occurred on the granulosa cells no such pattern was observed over the zona matrix. These studies indicate that the oocyte itself but not the granulosa cells elaborates the native immunogenic material of the zona pellucida. The administration of PMSG at particular stages of ovarian differentiation interferes with follicular development leading to an abnormal extracellular assembly of the zona and its degradation (phagocytosis) by the surrounding granulosa cells.