The rat MHC contains a distinct locus for Ia-like antigens. The alleles of this locus are not always associated with like alleles of the MLR and Ag-B (SD) loci in inbred strains. This is demonstrated by the Fischer strain, which is MLR and tag-B (SD) 1 (H-11) but shares an Ia specificity with BN (Ag-B3, H-1n). The biologic significance of anti-Ia is distinctive, having in vivo graft-enhancing capability, even when it is cross-reacting (i.e., Lew anti-Fi enhances BN). In contrast, anti-AG-B (SD) does not enhance in the rat model.