Mouse CD1. 1 is an MHC class I-like, non-MHC-encoded, surface glycoprotein that can be recognized by T cells, in particular NK1. 1+ T cells, a subset of αβ T cells with semiinvariant TCRs that promptly releases potent cytokines such as IL-4 and IFN-γ upon stimulation. To gain insight into the function of CD1. 1, a panel of nine mAbs was generated and used to biochemically characterize and monitor the surface expression of CD1. 1 on different cell types. CD1. 1 is a heavily glycosylated, β 2-microglobulin-associated surface protein. Its recognition by a panel of 12 Vα14-positive and-negative CD1-specific αβ T cell hybridomas was blocked by two groups of mAbs that bound to adjacent clusters of epitopes, indicating that different αβ TCRs bind to the same region of CD1. 1, presumably above the groove. Remarkably, CD1. 1 was mainly expressed by dendritic cells, B cells, and macrophages, suggesting a function in Ag presentation to Th cells. Furthermore, the cell type that expressed the highest levels of CD1. 1 was the splenic marginal zone B cell, a distinct subset of B cells that also expresses CD21 (the C3d receptor) and may be involved in natural responses to bacterial Ags. Altogether, the results support the idea that CD1. 1 may function in recruiting a form of innate help from specialized cytokine producer αβ T cells to APCs, a role that might be important at the preadaptive phase of immune responses to some microbial pathogens.