Mononuclear cell subpopulations from the peripheral blood (PB) and peritoneal fluid (PF) of fertile and infertile women were quantified by flow cytometry using a double-staining monoclonal antibody technique. No differences in the percentage distribution of mononuclear cells between fertile and infertile women were demonstrated when either the PB constituents or the PF components were compared to one another. When the mononuclear cell composition in the PB was compared with that in the PF however, the percentage of PF-activated T cells and monocytes was increased in both fertile and infertile women as opposed to that in the PB, while there was a decrease in T helper cells (fertile and infertile women) and natural killer cells (fertile women and infertile women with endometriosis). Even though shifts do exist in the immunocytes of PF from fertile and infertile women, evidence is lacking that the PF is an immunologic mediator of infertility.