Reflecting a growing awareness that China's heavy reliance on paid blood donors is contributing to the rapid spread of AIDS, a new law banning the buying and selling of blood took effect Thursday.

Medical centers all over China are bracing for disruptions. To meet rising medical needs, China's hospitals use a system in which poor people sell their blood, sometimes to middlemen who falsely declare that their products have been screened for disease.Paying for blood, rather than relying on voluntary donations, has been virtually banned in most countries because it promotes the spread of AIDS, hepatitis and other infections.

In part because of a strong cultural aversion to donating blood, China is unusually reliant on paid sources. According to official statistics, which some experts consider understated, 40 percent of all blood used in medical treatment in China has been sold.

Given the high dependence on blood sellers, the new law cannot be fully enforced for some time, senior blood bank officials admit.

The city of Beijing, for example, gets only half the blood it requires from local donations, and imports the rest from other regions of the country, said Zhang Lichuan, an official at the Fangshan Blood Station in a suburb of the city. Much of that presumably involved paid donors.

Hospitals will be forced to reduce the number of transfusions, officials say - as they have in the West in recent years, without ill effect.