Serological diagnosis of bullous pemphigoid is based on immunoblotting or indirect immunofluorescence on normal human salt-split skin. These methods are expensive or time-consuming and not available as a routine test in all laboratories. We used rat bladder epithelium as substrate for indirect immunofluorescence and compared it with other substrates and with immunoblotting. Twenty-nine bullous pemphigoid sera were studied on rat bladder epithelium, monkey oesophagus, salt-split skin and with immunoblotting on human keratinocyte cultures. Indirect immunofluorescence on rat bladder epithelium proved to be more sensitive (72%) than on monkey oesophagus alone (45%) and less sensitive than on salt-split skin (97%). Rat bladder epithelium, when tested on 41 sera of a control group, showed a very high specificity: 2/41 (95%). In combination with immunoblotting on keratinocyte extracts, indirect immunofluorescence on rat bladder epithelium allowed 93% of sera to be recognized, a value close to the salt-split skin alone. Rat bladder epithelium appears to be a more sensitive substrate than monkey oesophagus for the diagnosis of bullous pemphigoid and, although less specific, it is easier and faster than using salt-split skin, which remains indispensable to distinguish bullous pemphigoid from epidermolysis bullosa acquisita.