Only about 6 per cent of men who put themselves forward as possible sperm donors end up being accepted, according to Emma Hopson of the London Bridge Hospital's Fertility Centre. Only one-fifth of those who make inquiries are called for interview. At this stage, more fall by the wayside for medical reasons such as family histories of heart disease or allergy. Then, when semen is analysed, many men's sperm is often found to be of insufficient quality (too low in number, or of too limited motility) to survive the freezing process. This means that in most fertility clinics, there is always a slight shortage of donor semen, although no figures are available.

Since August 1991, 2,800 sperm donors have been registered in the UK. Those who volunteer their services are usually working men in the 20-34 age group, and not students, as is commonly assumed. Money is often an important factor in the decision to become a donor - they can be paid up to about pounds 15 a time for their services, plus travel expenses. However, Ms Hopson stresses that sperm donation also requires a firm commitment and a mature outlook. Those who are fit and healthy enough to be accepted as donors usually have to provide semen two or three times a week for a four- to six-month period.

All potential donors are interviewed to assess their medical and family history. Suitable candidates go on to be screened for hepatitis B, HIV and syphilis, and have their semen analysed. If a man's application is successful, the law requires him to give written consent for the use and storage of his semen. The clinic is obliged to offer counselling for any concerns he may have about his decision.

In the UK, donors and recipients are protected by the Human Fertilisation and Embryology Act of 1990, and treatment is regulated by the Human Fertilisation and Embryology Authority. The identities of donors remain confidential, except when the recipient of the sperm has intentionally chosen a donor known to her. But the HFEA does have a legal duty to tell people aged 18 or over, born as a result of donation, basic details (but not the name) of the donor if they ask, and those over 16 who wish to marry can be told if they could be related to their future spouse. To answer these questions, the authority keeps a confidential register of donors' dates of birth, eye and hair colour, occupation and interests.

HFEA regulations also state that donors should not be told of any ensuing pregnancies, nor told the identities of the recipients. The woman being treated and her partner are the legal parents of any child born, and the donor has no legal relationship to his offspring. The chance of a mating between a donor's children means that no more than 10 babies can be produced from the semen of one man, except when a recipient wants another child fathered by the same donor.