CD1 molecules are distantly related to the major histocompatibility complex (MHC) class proteins. They are of unknown function. Screening random peptide phage display libraries with soluble empty mouse CD1 (mCD1) identified a peptide binding motif. It consists of three anchor positions occupied by aromatic or bulky hydrophobic amino acids, Equilibrium binding studies demonstrated that mCD1 binds peptides containing the appropriate motif with relatively high affinity. However, in contrast to classical MHC class I molecules, strong binding to mCD1 required relatively long peptides. Peptide-specific, mCD1-restricted T cell responses can be raised, which suggests that the findings are of immunological significance.