Reaction of fetal tongue (A, B), adult medial rectus (C) and adult biceps brachii (E-L) with antibody (Ab) NCL-MHCn (A, D), Ab NCL-MHCd (B,E), Ab 4A6 (C,F), Ab N2.261 (G), Ab SC-71 (H), Ab 2F7 (I), Ab A4.74 (J), Ab MY-32 (K) and Ab A4.84 (L). Note reaction of Abs NCL-MHCn and NCL-MHCd to fetal tissue but not to biceps brachii, reaction of Ab N2.261 to biceps brachii MHCII fibers (Ab MY-32+/NCL-MHCn-) and reaction of Abs SC-71 and 2F7 to a subset of MHCII fibers in biceps brachii. Symbols *, o, •, +, denote the same fibers on serial sections. Calibration bar = 50μm.