The EASZY assay is a mechanism-based in vivo screening assay designed to detect endocrine active chemicals acting as agonist through estrogen receptors (ERs), by inducing the expression of the green fluorescent protein (GFP) driven by the cyp19a1b promoter. The EASZY assay allows for the detection of estrogenic activity of chemicals on transgenic tg(cyp19a1b:GFP) Zebrafish embryos exposed for 96 hours during the embryonic stages of development. At the end of the experiment, the fluorescence of each newly hatched eleutheroembryo is measured using fluorescence microscope. Because the skull of early developmental stages of zebrafish is transparent, GFP is observed, imaged and quantified in vivo. The intensity of fluorescence is then quantified using image analysis software.