Scientists say they've found an efficient way to make red blood cells from human embryonic stem cells.

The development is seen as a possible step toward turning out the elements for transfusions in the laboratory. That's important, because of donor shortages and disappointments in creating blood substitutes.

Researchers writing in the journal "Blood" say their cells act like natural red blood cells in lab tests, and their process can be used for large-scale production. The goal is to get embryonic stem cells to supply type O-negative -- or "universal donor" -- red cells for transfusion.

One expert calls the research "a very good start." The challenge now is to show that a complex lab process can mass produce red blood cells for transfusion, and that the cells can survive long enough in the human body to be useful.