Phenyl mercuric nitrate as an injection, in a dilution of 0-02% and buffered to pH 7, is a potent spermicidal agent which has been demonstrated to have no harmful side effects. It produces rapid death of all spermatozoa on contact. The injection of 2-5 ml of the solution into the lumen of each vas at the time of vasectomy is a simple procedure and adds no significant time and no complications to the operation. Studies have shown that no viable spermatozoa were present at seminal counts carried out one or two weeks after operation.