This memorial article describes Lord Florey's pioneering work on penicillin during World War II at Oxford's Sir William Dunn School of Pathology. Beginning in October 1939, Florey and Ernst Chain systematically investigated naturally-occurring antibacterial substances, focusing on Penicillium notatum. The team developed practical extraction methods using solvent transfer (ether/amyl acetate), established assay techniques, and optimized fungal culture conditions using modified Czapek-Dox medium with brewer's yeast extract. A crucial breakthrough occurred on May 25, 1940, when mice infected with streptococci and treated with penicillin survived while untreated controls died. This success prompted rapid scale-up using improvised equipment and novel ceramic culture vessels. By early 1941, Florey initiated human trials, with dramatic results in treating severe infections. Recognizing that laboratory production could not meet clinical demands, Florey traveled to America in July 1941 and established collaboration with the U.S. Department of Agriculture's Fermentation Division in Peoria. There, using corn-steep liquor as a growth medium supplement, researchers increased penicillin yield from 1-2 units/ml to 20 units/ml, laying the foundation for commercial production that would transform antibiotic therapy.

Key findings

• Florey and Chain developed a simple solvent-transfer extraction method using amyl acetate at different pH values to isolate and concentrate penicillin from culture fluid without requiring sterile solutions.
• Mouse protection experiments initiated May 25, 1940 demonstrated that penicillin treatment could save mice from lethal streptococcal infection, with surviving mice receiving as few as 200 total units.
• Early human clinical trials beginning February 1941 showed remarkable therapeutic potential in treating serious infections, though limited supply constrained treatment numbers.
• Collaboration with the U.S. Department of Agriculture's Peoria laboratory achieved a tenfold increase in penicillin yield by substituting corn-steep liquor for yeast extract in the culture medium.
• Florey's team employed innovative improvisation during wartime shortages, creating functional extraction equipment from repurposed materials and developing specialized ceramic culture vessels with manufacturers.