Three cases of allergic contact dermatitis due to N,N-dimethyl-para-phenylenediamine in a bacteriology laboratory are presented. This chemical, a dye closely related to the common contact allergen para-phenylenediamine, is used in laboratories to screen bacterial culture plates for Neisseria gonorrhoeae. The three patients had strongly positive patch test reactions to N,N-dimethyl-para-phenylenediamine, but had negative reactions to para-phenylenediamine. A brief discussion of allergic contact cross-sensitization among compounds with the para-aminophenyl grouping is presented.