The fine structure of human granulosa cells (GC) obtained in the course of an in vitro fertilization procedure was correlated with oocyte fertilizability. The investigation was performed by light-microscopic and electron-microscopic examination of ultrathin sections and replicas of freeze-fractured material. GC associated with nonfertilizable oocytes (group I) had significantly smaller cell areas, tended to be tightly packed, and exhibited abundant intercellular gap junctions and adherence junctions. Multiple structural variations of cytoplasmic organelles were observed in comparison to cells associated with fertilizable oocytes (group II). Cells in group II tended to be widely dispersed, frequently contained interiorized gap junctional elements (annular junctions), and showed morphologic correlates of high steroidogenic activity. Structural dissimilarities between groups I and II were not consistently related to follicle size, as determined by volume of aspirated follicular fluid. In view of the sequential ultrastructural changes known to occur during follicular maturation and ovulation, it is concluded that GCs related to nonfertilizable oocytes were lacking structural correlates of luteinizing hormone/human chorionic gonadotropin responsiveness.