"It is not wise to use penicillin as a prophylactic against every little infection."

"I have been very lucky in my scientific career. I have stumbled on things by accident."

"It is not difficult to make microbes resistant to penicillin in the laboratory by exposing them to concentrations not sufficient to kill them, and the same thing has occasionally happened in the body."

"A good many people think that I deserve some credit for the discovery of penicillin. They are wrong. It was discovered by nature."

"I had no idea that I had stumbled on to a subject that would prove to be of such immense importance."