Using affinity-purified antibodies against mouse liver gap junction protein (26 K), discrete fluorescent spots were seen by indirect immunofluorescence labelling on apposed membranes of contiguous cells in several mouse and rat tissues: pancreas (exocrine part), kidney, small intestine (epithelium and circular smooth muscle), Fallopian tube, endometrium, and myometrium of delivering rats. No reaction was seen on sections of myocardium, ovaries and lens. Specific labelling of gap junction plaques was demonstrated by immunoelectron microscopy on ultrathin frozen sections through liver and the exocrine part of pancreas after treatment with gold protein A. Weak immunoreactivity was found on the endocrine part of the pancreas (i.e., Langerhans islets) after glibenclamide treatment of mice and rats, which causes an increase of insulin secretion and of the size as well as the number of gap junction plaques in cells of Langerhans islets. Furthermore, the affinity purified anti-liver 26 K antibodies were shown by immunoblot to react with proteins of similar mol. wt. in pancreas and kidney membranes. Taken together these results suggest that gap junctions from several, morphogenetically different tissues have specific antigenic sites in common. The different extent of specific immunoreactivity of anti-liver 26 K antibodies with different tissues is likely due to differences in size and number of gap junctions although structural differences cannot be excluded.