ABSTRACT: The study of lymphocyte subsets from human follicular fluid (FF) provides an opportunity to evaluate immunological features of the ovary. We investigated the mononuclear cell subsets in FF and peripheral blood obtained at the time of laparoscopy from ten in vitro fertilization (IVF) patients. Midcycle nonpregnant peripheral blood was used as the control. A marked increase in the proportion of monocytes (CD14+) was observed in FF. Although FF was enriched with CD8+ lymphocytes, a decrease in the proportion of CD4+ lymphocytes was observed. “Memory” T cells in FF, identified by the CD4+ CD45R− phenotype, predominated over “naive” T cells (CD4+ CD45R+) at a ratio of 2:1, which differs from the ratio yielded by control blood samples (1:1). The percentage of activated T cells (CD3+ HLA-DR+ cell) increased significantly in FF. When lymphocyte subsets were studied in the peripheral blood of IVF patients, changes similar to but less significant than those in FF were found. These data support the concept that lymphocytes play an important role in ovarian physiology.