Evaluation of a predictive in vitro Leydig cell assay for anti-androgenicity of phthalate esters in the rat.

•An in vitro assay was developed for testosterone inhibition in R2C rat Leydig cells.•R2C results were highly consistent with in vivo inhibition data for phthalate esters.•The relative potency of 14 structurally diverse monoester phthalates was determined.•Some oxidative metabolites of monoethylhexyl phthalate also inhibited testosterone.•This assay is an in vitro alternative to rat studies for phthalate antiandrogenicity.