More than 1,000 blood donors are still needed in Sheffield to take part in a medical research project.
In the UK, the current waiting period between blood donations is 12 weeks for men, and 16 weeks for women.
During the two-year study men will be asked to donate every eight, 10 or 12 weeks and women every 12, 14 or 16 weeks.
The study aims to find the best time period between donations, to allow people to give more regularly and increase the supply of blood.
