In 1944, Margaret Jennings determined how penicillin acts, and showed that it has no lytic effects on mature organisms, including staphylococci; lysis occurs only if penicillin acts on bacteria during their initial stages of division and growth, when it interferes with the metabolic process that forms the cell wall. This brought Flemings explanation into question, for the mould had to have been there before the staphylococci. Over the next twenty years, all attempts to replicate Flemings results failed. In 1964, Ronald Hare took up the challenge. Like those before him, he found he could not get the mould to grow properly on a plate containing staphylococci colonies. He re-examined Fleming's paper and images of the original Petri dish. He attempted to replicate the original layout of the dish so there was a large space between the staphylococci. He was then able to get the mould to grow, but it had no effect on the bacteria.
Finally, on 1 August 1966, Hare was able to duplicate Flemings results. However, when he tried again a fortnight later, the experiment failed. He considered whether the weather had anything to do with it, for Penicillium' grows well in cold temperatures, but staphylococci do not. He conducted a series of experiments with the temperature carefully controlled, and found that penicillin would be reliably "rediscovered" when the temperature was below , but never when it was above . He consulted the weather records for 1928, and found that, as in 1966, there was a heat wave in mid-August followed by nine days of cold weather starting on 28 August that greatly favoured the growth of the mould.