Coronavirus patients who survived the deadly virus are wanted for an urgent trial.
Plasma donations are need for major research into how to treat those still suffering from the COVID-19 virus, including from people who were never tested.
The research is hoping to assess whether convalescent plasma donations can be transfused into patients who are struggling to develop their own immune response.
NHS Blood and Transplant (NHSBT), which is collecting the plasma for the trial, said people who were treated in hospital for Covid-19 produce the most antibodies, making them priority plasma donors.
More than three quarters of such patients had high enough antibody levels for their donation to be used in the national trial.
Just three in 10 (30%) Covid-19 patients who did not need hospital treatment had high enough antibody levels.
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But the body is urging anyone who had Covid-19 to come forward and donate - including men who had the symptoms but no test.
The plasma from former patients is rich in the antibodies that develop as a person recovers from an illness.
It is transfused into people who are seriously ill with Covid-19 and struggling to develop their own antibodies.
If the trial is successful, being treated with convalescent plasma could become a widespread practice in hospitals.
So far more than 13,000 donations have been taken, including more than 800 donations from people who were treated in hospital.
Dr Lise Estcourt, head of NHSBT's Clinical Trials Unit, said: "These figures demonstrate how important it is for people who were hospitalised with coronavirus to donate - they are most likely to be able to save the lives of others seriously ill people.
"These donors have higher antibody levels because while initially your immune system will try and fight off a virus with white blood cells, if you become more ill, your immune system needs to produce more antibodies that neutralise or kill the virus."
People who have had coronavirus are being encouraged to donate by calling or 0300 123 23 23 or visiting the www.nhsbt.nhs.uk website.
