Blood not so simple.
Red cells made in a laboratory have been infused into people.
The next step is to measure just how long the manufactured cells actually do last.
To that end, they have been tagged with a special radioactive dye commonly used in medicine to track things around the body.
If they do indeed outlive conventionally transfused cells, as the researchers hope and preclinical studies suggest they will, then recipients will not need such frequent transfusions.
That will help a lot.
At the moment, patients with blood disorders such as sickle-cell disease and thalassaemia may require a transfusion as often as every four to six weeks.
As a consequence, some develop iron overload, which causes severe complications.
Others end up forming antibodies against many blood types, which makes finding a matching donor harder.
If all goes well, the trial will be extended to include at least ten healthy volunteers.
But that is only the beginning.
Larger tests, including tests on actual patients, will be needed before this approach can be put into practice.
That will take time, for it normally requires between five and 15 years to introduce a new medical treatment.
Even then, the technique will probably be reserved for a favoured few - those possessing extremely rare blood types being at the head of the queue.
Unless some unforeseen breakthrough occurs, making the cells in quantity will be challenging.
At the moment, harvested stem cells eventually exhaust themselves, so the number of red cells a donation can yield is limited.
And manufacturing is a cottage industry.
Producing a batch of reticulocytes requires 24 litres of nutrient solution to generate a tablespoon or two of product.
The cost of scaling this up is unknown, but will probably be far more than the 145 pounds (166 dollars) that a normal blood donation currently costs in Britain.
It may eventually be possible to make the stuff in bulk.
But for now, human blood donors will continue to be extremely welcome.