The AMS-32.1 antibody reacts with the I-A[d] MHC class II alloantigen. It cross-reacts with cells from mice of the H-2[f], H-2[g7], H-2[i], and H-2[v] haplotypes. Reactivity with other haplotypes (e.g., k, p, q, r, s, u) has not been observed. Immunohistochemistry of I-A[d] positive cells. Frozen sections of mouse thymus were reacted with the anti-I-A[d] antibody. Cells expressing the MHC class II molecule I-A[d] can be identified by the intense brown labeling of their cell membranes. Magnification 20x.