Four patients allergic to many mammalian sera were found to have circulating, skin-sensitizing antibodies for these sera. A study of these antibodies by in vitro neutralization and subsequent local passive transfer showed that, in the case of each patient, one particular mammalian serum neutralized antibodies for all sera whereas other mammalian sera neutralized antibodies for themselves alone or for themselves plus one or several additional sera but not for all sera. The possibility of multiple common allergenic determinants in mammalian sera is discussed.