An increasing number of environmental chemicals is suspected to act similarly as endogenous estrogens at concentrations far below the so called "No Observed Adverse Effect Levels". In man, as in wildlife, these substances are held responsible for several reproductive disorders e.g. the decline in sperm quality and the increase in testicular cancer incidence in different countries worldwide. Several in vitro bioassays have recently been developed to screen single compounds for their ability to bind to estrogen receptors (ER) resulting in a hormone specific response. However, divergent results from these assays indicate the lack of validation and standardisation. Moreover, the proven cell specificity of estrogen response questions the relevance of the observed effects and necessitates the development of assays in a fertility related cellular context.