464 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
Further purification presented a harder problem, since penicillin 
is unstable toward many reagents normally used for purification pur- 
poses. It is unstable in dilute acids and alkalis and is destroyed by 
many metals, such as copper, cadmium, zinc, and mercury, and by 
primary alcohols and oxidizing agents such as potassium permanga- 
nate. This severely limited the selection of methods for purification, 
but penicillin preparations which are almost pure have now been 
obtained. 
The purest material obtained in Oxford has an activity of about 
1,000 units per milligram, corresponding to an inhibitory power 
against the staphylococcus of about 1 part in 50,000,000. Pure crystal- 
line sodium penicillin has been obtained. It contains about 1,670 Ox- 
ford units per milligram. By international agreement the Interna- 
tional Unit is the specific penicillin activity contained in 0.6 
microgram of pure penicillin II (or G). The unique feature of 
penicillin, when compared with other antibacterial substances, is 
that it combines this astonishingly high antibacterial activity with 
a very low toxicity. An amount several hundred times greater than 
the therapeutic dose can be injected intravenously—into mice—with- 
out any noticeable toxic effect. 
This lack of toxicity is also observed with less pure preparations, 
and it may indeed be counted as extremely fortunate that none of the 
numerous contaminating substances causes harmful effects, even when 
the dose of penicillin is large—far beyond that necessary for thera- 
peutic purposes. For this reason it is unnecessary to subject peni- 
cillin to the elaborate and difficult purification processes before it can 
be used for clinical purposes. 
After the low toxicity of penicillin had been demonstrated on mice, 
a more extended study of its biologic properties was undertaken. It 
was shown that a concentration at least a hundred times greater than 
that necessary to stop the growth of sensitive bacteria was harmless to 
the white cells of the blood and to tissue cells grown in glass vessels. 
The very low toxicity of penicillin to the white cells of the blood was 
of particular importance, as these cells play an important part in the 
defense of the body against invading bacteria, which they have the 
power to ingest and destroy. 
Further research has shown clearly that when bacteria are put in 
nutrient media in which they can divide, then penicillin will kill them. 
If they are in such a condition that they cannot divide, then penicillin 
does not kill them. 
It was further shown that penicillin is readily absorbed from an 
intramuscular or subcutaneous injection and from the small intestine. 
Once absorbed in sufficient quantity, a simple test demonstrates that 
it is present in the circulating blood. Unfortunately it cannot be 
given by mouth, because the acid of the stomach would destroy it 
