We studied sera from 59 patients with bullous pemphigoid (BP) and 25 control subjects (normal volunteers, patients with pemphigus, psoriasis, eczema, or other dermatoses) by western blotting analysis on protein bands from normal human heat-separated epidermis. BP sera reacted with four protein bands that were not detected by control sera: two major bands at 220-240 and 165 kD and two faint bands at 190 and 95 kD. Three of these bands were significantly associated with BP: 220-240 kd (51% of the BP patients; p less than 0.001), 165 kD (49%; p less than 0.001) and 190 kD (20%; p less than 0.05). These results are consistent with a molecular heterogeneity of BP antibodies, because each individual BP serum showed a distinctive pattern of reactivity. Thirty out of the 59 BP sera contained anti-basement membrane zone antibodies demonstrable by indirect immunofluorescence (IIF). All these IIF positive BP sera reacted by immunoblotting with at least one protein band: 23 (77%) with the 220-240-kD band and 21 (70%) with the 165-kD band. Furthermore, 45% of the 29 IIF negative BP sera showed a reactivity with the 220-240-kD band and/or the 165-kD band. These results indicate that western immunoblotting might be a more sensitive method for the detection of circulating BP antibodies than IIF techniques, including IIF on salt split skin.