Endometritis in breeding cattle occurs during the postpartum period, and is associated primarily with contamination of the reproductive tract involving Arcanobacter pyogenes (formerly Actinomyces pyogenes) together with Gram-negative anaerobes. Polymorphonuclear inflammatory cells (PMNs) contribute partly to the defense mechanisms against micro-organisms contaminating the vagina and uterine lumen, whose phagocytic activity depends on bacterial opsonisation by humoral antibodies; significant numbers of lymphocytes are also present. Whilst leukocyte numbers in the uterine lumen are relatively high during metoestrus and dioestrus compared to other phases of the oestrous cycle, their functional activity is unaffected. Humoral antibody concentrations in the reproductive tract are stimulated following exposure to local antigen, and the response is site dependent; of the several different classes of immunoglobulins, IgG predominates in the uterus and IgA the vagina. Only a portion of the total IgG1 found on the uterine lumen is synthesised locally in the endometrium, the remainder and all of the IgG2 is derived from the local uterine blood supply. Generally, concentrations of immunosuppressant proteins present in the uterine lumen increase under progesterone dominance, and these inhibit lymphocyte proliferation, making the uterus more susceptible to infection. The relationship between uterine susceptibility to micro-organism contamination and the luteal phase of the oestrous cycle is still unclear. Intrauterine infusion of immunomodulators such as E. coli lipopolysaccharides (LPS) or oyster glycogen, in healthy cows and those with endometritis, stimulates leukocytes to migrate into the uterine lumen. At a dosage rate of 100 microg, lipopolysaccharides are not absorbed by the healthy endometrium and do not alter the oestrous cycle length. It is unknown, whether a similar dose can be absorbed through an inflamed endometrium in naturally occurring cases of endometritis to cause systemic illness. Currently, prostaglandin F2alpha is recommended for treating endometritis in both cycling and non-cycling cows, but its mode of action in non-cycling cows is not fully understood. The efficacy of endometritis treatment using an intrauterine infusion of an immunomodulator in cases occurring naturally has not been determined on a large scale.