In some aged MRL/Ipr autoimmune disease-prone mice, unexpected reactivity has been observed between lymphoid cells and mAb or polyclonal antibodies directed against the A beta d class II chain of the MHC. However, Southern blot analysis of their genomic DNA, using different class I and class II MHC-specific probes, confirmed their inbred character and their H-2k genotype. In this study, immunoprecipitation experiments with an anti-A beta d mAb indicated that the 45 (and 12)-kD molecule(s) recognized by an anti-A beta d mAb differed from a class II chain. After specific antibody screening of a lambda gt11 expression library constructed with MRL/Ipr "A beta d-positive" lymphoid cells, we cloned cDNA that share sequences with high homology (> 80%) to the 3' end of a 7-kb L1Md (L1) element propagated in the mouse genome via retrotransposition. Northern blot analysis showed an overtranscription of these L1 sequences in the MRL/Ipr spleen cells used for the construction of the cDNA library, in comparison to the MRL ancestor strains. Therefore, the autoimmune MRL/Ipr strain could express a translation product of L1 open reading frame 2, which mimics a "highly antigenic" epitope of an allogenic MHC class II Ag.