To evaluate the functional status of endometriotic implants, a luteal secretory product of endometrium, PRL, was measured in the PF of 27 infertile women with endometriosis, 13 infertile women without endometriosis, and 11 fertile women undergoing elective sterilization. PF PRL concentrations and PF/plasma PRL ratios were similar in all the groups and did not vary with the menstrual cycle or the stage of endometriosis. These data demonstrate that the ectopic endometrium in infertile women with endometriosis does not secrete PRL in amounts sufficient to elevate PF concentrations and suggest that the infertility observed in these women is probably not dependent upon the secretory activity of endometriotic implants.