The M5/114.15.2 monoclonal antibody recognizes a polymorphic determinant shared by the I-A[b], I-A[d], I-A[q], I-E[d], and I-E[k] (but not I-A[f], I-A[k], or I-A[s]) MHC class II alloantigens that can be expressed by B cells, dendritic cells, monocytes, macrophages and activated T cells. It also reacts with cells from mice of the H-2[p] and H-2[r] haplotypes, and it is non-reactive with cells from NOD (H-2[g7]) mice. Flow cytometric analysis indicates that the M5/114.15.2 and 2G9 monoclonal antibodies have comparable reactivity on cells from mice with I-A[b], I-A[d], I-A[g7], I-A[q], I-E[d], and I-E[k] alloantigens.