Abstract
In a paper published by one of us last year we called attention to the fact that it was quite likely that not all the toxic substances produced in cultures by bacteria can be peremptorily classified as either exotoxins or endotoxins. The writers working with a number of different organisms, biologically unrelated, have found poisonous substances of moderate potency developed both on fluid and on solid media which they believe should not be regarded at the present time either as specific or antigenic exotoxins, or as endotoxins. Indeed, it seems quite impossible at the present time to definitely classify these substances, for which reason they are referred to in our laboratory, for the present, as the “X” substances.
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