Mycolic acids (MAs) are highly hydrophobic long-chain α-alkyl β-hydroxy fatty acids present in the cell wall of Mycobacterium tuberculosis (Mtb) as a complex mixture of molecules with a common general structure but with variable functional groups in the meromycolate chain. In this study, we addressed the relationship between the MA molec-ular structure and their contribution to the development of T-cell immune responses. Hereto, we used the model antigen ovalbumin and single synthetic MAs, differing in oxygenation class and cis versus trans proximal cyclopropane configuration, as immune stimulatory agents. Subcutaneous delivery of liposome-formulated MAs with a proximal cis cyclopropane elicited antigen-specific Th1 and cytotoxic T-cell immune responses, whereas intratracheal immunization elicited pulmonary Th17 responses. These immune stimulatory activities depended not only on the cis versus trans proximal cyclopropane configuration but also on the MA oxygenation class. Our study thus shows that both the presence and nature of the functional groups in the meromycolate chain affect the immune stimulatory adjuvant activity of Mtb mycolates and suggests that Mtb bacilli may impact on the host protective immune response by modulating the cis versus trans stere-ochemistry of its mycolates as well as by altering the oxygenation class of the meromy-colate functional group. ; J.G. is supported by the UGhent Concerted Research Consortium BOF12/GOA-B/12424/01 and the Interuniversity Attraction Pole consortium IUAPVII/63. J.G. and K.H. are supported by the TBVAC2020 consortium from the EU H2020 program. J.G. and J.V. are supported by an FWO-Flanders and NRF-South Africa bilateral research grant. M.S. is supported by an IWT grant. ; http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1521-4141 ; 2017-09-30 ; hb2016 ; Biochemistry