Outside the Botanic Garden, between Danby Arch and the High Street, is a small stone memorial that commemorates the work done in Oxford on the isolation and purification of the antibiotic drug penicillin. The adjacent rose garden was planted in the researchers' honour.

Alexander Fleming's observation, on one of his culture plates, of a mould that killed bacteria led to the wartime development and production, initially in Oxford and later in America, of penicillin. Howard Florey, a quiet Australian pathologist, and Ernst Chain, an ebullient German-born biochemist, provided a contrasting but complementary leadership to the Oxford team. Florey and Chain's team began to isolate and purify the penicillin as war broke out. With the first precious samples they began to get results, first on mice and then on an Oxford policeman who had terrible absceses and boils. The team had nothing to lose by trying the drug on him because the man would otherwise have died. He was administered the drug by Dr Fletcher. The policeman's illness went into remission, and on the fifth day he was sitting up in bed. However, the penicillin then ran out. Ingeniously the team extracted the drug from his urine, but there was not enough to go on treating him, and he died. However, it was clear that the drug was effective.

As a contingency plan in the event of enemy invasion, the precious penicillin was to be smeared on the researchers' coat linings so there would be a chance of them escaping and restarting the culture elsewhere. Norman Heatley discovered that the best vessel in which to grow the mould was a hospital bedpan. They borrowed sixteen from the Radcliffe Infirmary and then went to a firm in the Potteries, where a modified form of bedpan was made to Heatley's specification. However, the manufacture and large-scale production of penicillin was eventually done in the USA, and sufficient quantities were ready in time to treat casualties in the Normandy landings. Florey and Chain, together with Fleming, won the 1945 Nobel Prize for Physiology and Medicine.

Alongside the rose garden is the Daubeny building. Now used mainly as student accomodation, it was originally a laboratory, built at his own expense by Charles Daubeny, Professor of Botany from 1834 to 1867. He was responsible for a major rearrangement of the Botanic Garden. The inscription above the entrance to the building, Sine experientia nihil sufficienter sciri potest (Without experience nothing can be known sufficiently), is from Roger Bacon's Opus Majus, written in 1267-8.