2,4-dinitrophenylsulfenyl chloride (DSCl) and 2,4-dinitrophenylthiocyanate (DSCN) elicited allergic reactions of the delayed type when applied to the skin of guinea pigs and of human beings who had been sensitized by prior exposure to 2,4-dinitrofluorobenzene (DF). DSCl and DSCN, together with 2,4-dinitrobenzene sulfonate (DSO₃), constitute a clearly defined group of allergenic dinitrophenyl compounds in that they all combined with skin protein in vivo through reaction with cysteine or cystine. In vitro, these compounds combine with free SH groups, and with —S—S— groups of hair and epidermis, but not with —S—S— groups of oxidized glutathione or of bovine gamma globulins. DSO₃, DSCl, and DSCN did not react with amino groups in vivo, but did react with protein amino groups in vitro at pH values of about 10.

Another group of dinitrophenyl compounds (DF, DCl, and DBr) previously had been shown to combine with lysine ϵ-NH₂ groups of epidermal proteins. In the present work it was found that these compounds do not react with the disulfide groups of these proteins, either in vivo or in vitro. Moreover, they did not seem to react with SH groups of viable skin, although they are highly reactive with sulfhydryl in vitro. This apparent discrepancy between reactivity with SH groups in vitro and in vivo may be due to the fact that the chromatographic technique employed was relatively insensitive for the sulfhydryl derivative.

When a compound of either group was applied to the skin surface, dinitrophenyl-amino acids were recovered from the epidermis but not from the dermis.

The results are discussed from the viewpoint of the epidermal localization of dinitrophenyl-protein conjugates.