Natural and synthetic compounds called endocrine-disrupting chemicals can interfere with normal hormone binding to convey inaccurate signals or send mixed messages that may result in altered health outcomes of both wildlife and humans. Estriol is one of natural origin endocrine disruptors and it is metabolite of 17β-estradiol. The aim of this work was to develop a method for determining free estriol available to capaciting sperm. In order to determine a status of estriol during mouse sperm capacitation in vitro a high performance liquid chromatography HPLC method with UV detection was used. A free estriol, and the estriol bound to the bovine serum albumin in capacitation medium can be quantified by the proposed method. A reversed-phase separation mode using a SunFire C18 column with a simple mobile phase composed of acetonitrile and water, methanol and water at the ratio 40/60 (v/v) was applied. Our results show that the level of free estriol available for mouse spermatozoa during capacitation in vitro can be quantified by HPLC method with UV detection. Therefore, this method represents an important tool to determine the amount of environmental estrogens, such as estriol, bound to sperm cells at the specific time point of capacitation in vitro.