We used a ‘hit and run* gene targeting strategy to generate mice expressing only the~ 31 isoform of the consewed invariant (Ii) chain associated with major histocompatibility complex (MHC) class II molecules. Spleen cells from these mice appear indistinguishable from wild type with respect to class II subunit assembly, transport, peptide acquisition, surface expression, and the ability to present intact protein antigens. Moreover, these mutant mice have normal numbers of thymic and peripheral CD4+ T cells, and intact CD4+ T-dependent proliferative responses towards a solubleantigen. In short, MHCclass II expression and function are surprisingly unaffected in mice lacking p41 invariant chain, implying that the p31 and p41 isoforms may be functionally redundant in the intact animal.