Isobaric labeling reagents such as Tandem Mass Tags (TMT®) enable the genome-wide quantification of protein expression levels under different conditions using a gel-free MS/MS-based approach. Here, we applied a TMTduplex approach with two isobaric tags to study the response of the human pathogen Neisseria meningitidis to deprivation of iron, a condition met in the human body. In total, 609 proteins were identified in samples of three independent growth experiments, in which we compared cultures grown in the presence and absence of iron. Expression of 35 proteins was found to be induced or repressed under iron-limiting conditions, including 11 proteins whose ORFs were not previously identified in DNA array studies as being regulated by iron availability at the transcriptional level. These 11 proteins include proteins likely involved in iron metabolism. ©2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.