Ocular cicatricial pemphigoid (OCP) is a chronic autoimmune inflammatory disease which affects the conjunctiva and other squamous epithelial mucous membranes resulting in a scarring process. It is characterized by the deposition of an anti-basement membrane zone (BMZ) antibody in vivo. Sera from 11 patients with active OCP were studied. Using monkey esophagus and normal skin as substrate, weak staining of the BMZ was observed in conventional indirect immunofluorescence (IIF) assay. Using salt split human skin as substrate, the OCP sera demonstrated binding to the epidermal side of the split, in low titers with weak staining. Ten of the 11 sera were positive on an immunoblot assay using COLO and SCaBER tumor cell lysates demonstrating 230, 205, 160, and 85 kD proteins. Sera from six bullous pemphigoid (BP) patients, with only cutaneous involvement and high titer of anti-BMZ antibody, as detected by IIF, also bound to 230, 160, and 85 kD proteins on both lysates in comigration experiments. Serum from five normal individuals and two patients each with severe atopic conjunctival disease, erythema multiforme with chronic conjunctivitis and systemic lupus erythematosus (SLE), did not demonstrate those bands. When the two lysates were first absorbed with BP sera and then the same lysates were immunoblotted with OCP sera, in all ten OCP sera the 230, 160, and 85 kD bands were eliminated and only a single 205 kD band was uniformly present. These results indicate that OCP sera recognize peptide(s) present in 230, 205 and 160 kD proteins in lysates from COLO and SCaBER tumor cells. These proteins contain the immunodominant region of the BMZ molecule(s) in which the OCP antigen(s) reside. The OCP antigen(s) appears to be distinct from the BP antigen(s).