Straub offered a direct challenge to Ehrlich's theory of so-called chemoreceptors. At the annual assembly of German naturalists and doctors in 1912, Straub pointed out that there were many pharmacologically active substances (such as nitrous oxide, carbonic acids and kali salts) whose constitution made it very unlikely that they were capable of reacting chemically within the organism. There might be receptors for certain poisons, he conceded, but to build a whole theory of chemoreceptors on this was impermissible [19].