Competitive protein-binding assays for steroid hormones were conducted on human semen. Mean concentrations for 10 specimens from 5 healthy young men were: testosterone 6.8 ng/ml, oestradiol-17β 9.5 ng/ml, progesterone 0.6 ng/ml. Oral administration of 0.5 g daily chlortetracycline for 7 days gave a semen tetracycline mean concentration of 4.5 μg/ml. Washed sperm bound tritiated steroid hormones in vitro much better than sperm in diluted semen. Testosterone reduced sperm uptake of 3H-oestradiol-17β, while oestradiol-17β reduced uptake of both 3H-testosterone and 3H-progesterone. Pre-incubation of washed sperm with either oestradiol-17β or testosterone increased tetracycline-binding capacity, while progesterone stimulated tetracycline release. It is suggested that steroid hormones in male and female genital secretions modify the fertilizing capacity of human sperm.