Mouse epidydimal sperm cells have the spontaneous ability to take up exogenous DNA, a part of which is further internalized into nuclei. We report here that sperm cells from MHC class II knockout mice have a reduced ability to bind DNA compared to sperm cells from wild-type animals. Spermatozoa from CD4 knockout mice are instead fully capable of binding exogenous DNA, yet lose the ability to further internalize it. MHC class II expression was not detected on sperm heads using monoclonal antibodies. In contrast, CD4 molecules were found on sperm heads by both immunofluorescence and Western blot analysis. Moreover, we show that nuclear internalization of exogenous DNA was prevented in wild-type sperm cells preincubated with anti-CD4 mAbs. These results support the conclusion that CD4 and MHC class II molecules play distinct roles in the process of sperm/DNA interaction: though not present in mature sperm cells, MHC class II expression appears to be required during spermatogenesis to produce sperm cells capable of taking up foreign DNA, while CD4 molecules present on sperm cells mediate the nuclear internalization of sperm-bound DNA.