This collaborative work was undertaken to resolve discrepancies in reports of the number of forms of complement component C6 present in the circulation of mice from various inbred strains. Plasma C6 was analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and by isoelectric focusing (IEF), and C6 band patterns were developed by electroblotting and immunoprobing. Results of C6 allotyping of mice from 36 strains confirmed that while 20 strains (prototype strain BALB/c) possessed only one relative mass (Mr) for which typed C6A1 on IEF, the other 16 strains all possessed more than one C6 Mr form. Moreover, IEF analysis demonstrated additional polymorphic differences; among these 16 strains, 11 typed C6A1B1 like the prototype strain CBA, the AKR and RF/J strains typed C6A2B2, and the Japanese MOM strain as well as the C57BR/cdJ and C57L/J strains possessed two forms with IEF mobilities intermediate between C6A1B1 and C6A2B2. These will now be referred to as C6A3B3. Thus, a total of four different mouse C6 haplotypes have been identified. Testing C6 allotypes in a limited number of wild mice revealed that haplotypes found in inbred strains of Western or Eastern origin tend to reflect haplotypes of the wild mice from Europe or Japan, respectively.