ABSTRACT: Endometrial lymphocytes were isolated in two groups of fertile women, 17 of them normal and seven with various clinical problems. In order to study their immunological properties, endometrial and blood lymphocytes were cultured with either phytohemagglutinin (PHA), concanavalin A (Con A), pokeweed (PW), or purified protein derivative (PPD). Lymphocyte response was assayed by tritiated thymidine incorporation. The percent of T cells was estimated by E-rosette formation. In the normal fertile group, endometrial lymphocyte response to PHA and to Con A was found to correlate with the days of menstrual cycle, rising in a linear fashion between days 9 and 24. In the group with clinical problems, no correlation was noted. In neither group were endometrial lymphocyte responses to PW or PPD or percent of endometrial lymphocyte rosette formation influenced by the menstrual cycle. The modulation of endometrial lymphocyte response to PHA or to Con A stimulation in normal fertile women during the menstrual cycle seems unrelated to the number of T cells.