About 10 to 20 percent of the people who have registered with the Shanghai Stem Cell Donation Bank balk when asked to make a donation, officials with the center said.

They said donating stem cells isn't dangerous or harmful and those who sign up with the bank should follow through with their offer to help people in need if asked.

Some 100 people suffering from leukemia have found stem cell matches since the bank was established in 1992, but less than 40 surgeries have been performed so far.

Officials say some people cannot get a transplant because their physical situation is too poor to afford the operation, which costs 200,000 (US$24,096) to 300,000 yuan.

"What's more, we lose contact with some donors after they change their addresses or phone numbers, and some refuse due to personal reasons like health problems and plans to have a baby," said Sun Hongying of the Shanghai Red Cross, which operates the bank. "Some are just afraid of possible damage to their own health."

Medical experts said stem cell donation is a mature and safe technology, and donors aren't in any danger and they don't suffer any pain. Donors fully recover from the operation within a week.

"They shouldn't let those patients down when a donation is their last chance," Sun added.

Officials have expressed concerns about schools and companies organizing groups to register with the stem cell bank.

"Since many donors sign up on the spur of the moment or are simply following others, they don't think the process through clearly," said Zhang Jin, a Red Cross official. "Since the bank has to pay 500 yuan for a blood check for every registered donor, we want all donors to stay in contact and promise to help if needed."

The city's stem cell bank is the biggest in the nation with some 43,000 registered donors.