Florey’s most important work was turning penicillin into the first antibiotic. He worked with Ernst Chain to explore discoveries made earlier by Alexander Fleming. They looked at antibacterial substances that came from bacteria and fungi. They found that penicillin was the most interesting. Fleming had first discovered it in 1928. They were able to start experiments on humans in 1941. Because of World War 2 the drug was made in the USA. Its affects on wounded people was described as "miraculous".