Experiments on the sensitization of guinea pigs with simple chemical compounds are described. Positive effects were obtained by the administration of small quantities, namely fractions of milligrams, with 1:2:4 chlorodinitrobenzene, p-nitrosodimethylaniline, 1:2:4 trinitrobenzene, picryl chloride, four dichlorodinitrobenzenes, and a number of other aromatic compounds. Several substances chemically similar to those enumerated gave negative results. The first named compound is known to produce hypersensitiveness in human beings, a large number of cases having been observed in factory workers.

The mechanism of these effects is discussed.

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