PENICILLIN—FLOREY AND CHAIN 463 
activity was retained in the culture medium for some weeks. This 
suggested that, if appropriate conditions could be found, the extrac- 
tion of penicillin from the culture medium and its purification would 
be possible. 
For work of the scope envisaged it was apparent that results would 
be obtained most quickly by a team of workers, and we have been par- 
ticularly fortunate in our collaborators. Dr. N. G. Heatley devised 
a simple and quick quantitative assay method which has proved in- 
valuable in the elaboration of purification processes for penicillin and 
in similar investigations on other antibiotics. He also designed and 
constructed the first large-scale laboratory plant for. growing the 
mold and extracting penicillin. Dr. A. G. Sanders later devised 
and built alternative apparatus for the extraction of penicillin on a 
larger scale. Prof. A. D. Gardner of the Department of Path- 
ology, Oxford, collaborated throughout on the bacteriologic aspects 
of the work. The biologic investigations were carried out by H. W. 
Florey in collaboration with Dr. M. A. Jennings of the School of 
Pathology, Oxford; the chemical and biochemical investigations by 
E. Chain in collaboration with Dr. E. P. Abraham. The therapeutic 
trials on man were conducted first by Dr. C. M. Fletcher and later 
by Dr. M. E. Florey, with the help of many physicians and surgeons. 
It was established that penicillin was an acid of low molecular 
weight, which was stable in water. In an acid medium, it was found 
to be quickly destroyed. It could, however, be extracted by various 
organic solvents, such as ether, chloroform and amyl acetate, from 
acid solution and was found to be quite stable in these solvents. 
From the organic solvents it could be re-extracted into water by 
the addition of the right amount of alkali. Little loss of antibac- 
terial activity occurred during these operations provided they were 
carried out quickly and the solutions were kept cold. This transfer of 
penicillin between solvents and water has become the basis of all the 
extraction processes used on a large scale by commercial firms. 
By repeating the process several times and varying the solvents, a 
considerable purification and concentration of penicillin is achieved. 
On drying the final solution from the frozen state a preparation of 
penicillin is obtained in the form of a yellow powder which keeps its 
antibacterial activity unchanged for a long time. Though chemically 
still very impure—these preparations contain only about 10 to 20 
percent of pure penicillin—the antibacterial power is great. They 
contain about 100-200 Oxford penicillin units per milligram, which 
means that when diluted from 1 part in 5,000,000 to 1 part in 10,000,- 
000 they prevent the growth of staphylococci. Preparations of this 
degree of purity can be used for all clinical purposes. 
