250 Discussion 



and i5onicotinic acid. I am not satisfied that this occurs to the same 

 extent in media in which organisms are growing, because firstly it requires 

 quite a considerable amount of oxygen to occur at all. Secondly, the 

 process is rather slow. We found in some of our experiments, in which 

 we added haemin at intervals to media which had got isoniazid in them, 

 that one could reverse the action of isoniazid if the haemin was added 

 immediately, but after a day or two the effect of the haemin got less and 

 less. In other words, by that time the isoniazid must have caused some 

 irreversible changes. So that you do, in fact, get growth of sensitive 

 cells in the presence of haemin ; when you subculture them (they grow 

 in 10 or 50 (i.g. /ml., depending on how much haemin you have there) 

 they are found to be quite sensitive. 



