Two populations of reduced subunits were present in the mucins purified from pooled normal secretions and asthmatic and chronic bronchitic sputa; their relative level differed between samples. To investigate the nature of this heterogeneity, an asthmatic respiratory mucin preparation from a single individual was reduced and alkylated with 14C-iodoacetamide. This preparation was analyzed by gel filtration, agarose gel electrophoresis, immunoblotting, rate-zonal- and density-gradient centrifugation, and HPLC ion-exchange- and reverse-phase chromatography. Two populations (A and B) of reduced mucin subunits and a high-M(r)protein-rich fraction were identified. Species A has the higher molecular mass, is slowest migrating on agarose electrophoresis, has longer oligosaccharide chains, and expresses the carbohydrate structure sialyl-Le(x). Species B has a lower molecular mass, migrates faster in agarose electrophoresis Species B has a lower molecular mass, migrates faster in agarose electrophoresis, has shorter chains, and does not express sialyl-Le(x). The two subunits have similar but not identical amino acid compositions and 14C-tryptic peptide maps indicating they have different protein cores. The anti-sialyl-Le(x) antibody selectively precipitated subunit A not only from the reduced but also from the nonreduced mucin preparation, demonstrating that subunits A and B are present in different intact mucins.