Endometrial biopsies were obtained from four mares during consecutive oestrous cycles on the first day of oestrus, on the day when ovulation was detected, and four and eight days after ovulation. Cycle stages were confirmed by means of rectal palpation, ultrasonography and plasma progesterone determination. Immunohistochemical evaluation of the formalin fixed biopsy specimens was performed using a peroxidase anti-peroxidase technique. Immunoglobulin (Ig)A-, IgM-, IgG(Fc)- and IgG(T)-containing cells were detected in all biopsies; with IgA- and IgG(Fc)-containing cells generally predominating. There was no cyclical trend of Ig-containing cell numbers for any isotype. Free immunoglobulins of the four classes evaluated were frequently seen in luminal epithelium, glandular epithelium and secretions, and interstitium. This study of endometrial biopsies from a limited number of cycling mares suggests the presence in the equine endometrium of free and intracellular immunoglobulins of the classes A, M, G(Fc) and G(T) without any apparent cyclical trend.