The endocrine and reproductive effects of xenobiotics are believed to be due to (1) their mimicking the effects of endogenous hormones; (2) their antagonizing the effects of endogenous hormones; (3) their altering the pattern of synthesis and metabolism of natural hormones; and (4) their modifying hormone receptor levels. It has been suggested that endocrine disruptors may play a role in the decrease in human semen quantity and quality, an increase in the anomalies of male genital tract, and an increase in the testicular and breast cancer incidence during the last 50 years. Testing these hypotheses will require: (1) identifying estrogen and androgen agonists and antagonists among the chemicals present in the environment; (2) assessing the interactions among the endocrine disruptors to which humans are exposed; and (3) finding markers of estrogen (and androgen) exposure. The development of fast and sensitive bioassays is central to the achievement of these three goals.