Classical class I molecules present endogenous peptide antigens for recognition by cytotoxic T lymphocytes, which are transported by a heterodimer encoded by the TAP1 and TAP2 genes. Nonclassical class I (class Ib) molecules, although similar in structure to classical class I molecules, have an unknown function. We are interested in studying the cell biology and biochemistry of the representative murine class Ib molecules, Q7/b and mCD1.1, with the ultimate goal of determining whether nonclassical class I molecules can present viral antigens to cytotoxic T cells. To study these proteins in a variety of tissue culture cell lines in a strictly controlled manner in vitro and in mice we produced vaccinia virus recombinants expressing these proteins. Using these recombinants we found that cell surface expression of both Q7/b and mCD1.1 requires assembly with beta2-microglobulin. Q7/b expression also requires cells to express TAP, while mCD1.1 expression is TAP independent. In collaboration with A. Bendelac (Princeton University) we found the antigen presentation function of mCD1.1 to a unique subset of T cells parallels the requirement for surface expression.