MAJOR histocompatibility complex (MHC) class I and class II molecules bind immunogenic peptides and present them to lymphocytes bearing the αβ T-cell antigen receptor (TCR)^1–4. An analogous antigen-presenting function also has been proposed for the non-MHC-encoded CD1 molecules⁵, a family of non-polymorphic, β₂-microglobulin-associated glycoproteins^5–8 expressed on most professional antigen-presenting cells^9–11. In support of this hypothesis, CD1 molecules are recognized by selected CD4⁻CD8⁻ αβ or γδ8TCR⁺ T-cell clones^12–14, and we have recently shown that CD1 molecules restrict the recognition of foreign microbial antigens by αβTGR⁺ T cells¹⁰. But the substantial structural divergence of CD1 from MHC class I and class II molecules⁷, raises the possibility that the antigens presented by the CD1 system may differ fundamentally from those presented by MHC-encoded molecules. Here we report that a purified CDlb-restricted antigen of Mycobacterium tuberculosis presented to αβTCR⁺ T cells is mycolic acid, a family of α-branched, β-hydroxy, long-chain fatty acids found in mycobacteria^15,16. This example of non-protein microbial antigen recognition suggests that αβTCR⁺ T cells recognize a broader range of antigens than previously appreciated and that at least one member of the CD1 family has evolved the ability to present lipid antigens.