He discovered that by separating red blood cells from plasma, blood could be preserved and reconstituted at a later date. He researched blood transfusions, developed improved blood storage techniques, and applied his specialized knowledge to the development of large-scale blood banks at the beginning of World War II. This enabled doctors to save thousands of Allied lives. As the most prominent African American in the field, Drew protested the practice of racial segregation in blood donation because it lacked scientific basis. He resigned from his position with the American Red Cross who maintained the policy until 1950.