T cell hybridomas isolated from nonresponder H-2 b mice immunized with pork insulin were stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched antigen presenting cells. The restriction element used by these CD4− T cells was mapped to an oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1 b using transfectants. The antigenic determinant was localized to the insulin B chain, and experiments with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1 b–restricted T cells does not require transporters associated with antigen processing (TAP), and it is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present soluble insulin to Qa-1 b–restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1 b can function to present exogenous protein to T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may have access to a novel antigen processing pathway that is not available to class Ia molecules.