The mouse model (Cftr^tm1UNC /Cftr^tm1UNC) for cystic fibrosis (CF) show mucus accumulation and increased Muc1 mucin mRNA levels due to altered splicing (Hinojosa-Kurtzberg et al., (2003) Am. J. Gastrointest. Liver Physiol. 284, G853-62). However, it is not known if Muc1 is a major mucin contributing to the increased mucus and why CF/Muc1-/- mice show lower mucus accumulation. To address this, we have purified mucins from the small intestine of CF mice using guanidinium chloride extraction, ultracentrifugation and gel filtration and analyzed them by slot blot, gel electrophoresis, proteomics and immunoblotting. Normal and CF mice with wild type Muc1, Muc1-/- or that are transgenic for human MUC1 (MUC1.Tg, on a Muc1-/- background) were analyzed. The total amount of mucins, both guanidinium chloride soluble and insoluble, increased up to 10 fold in the CF mice compared to non-CF animals whereas the CF mice lacking Muc1 showed intermediate levels between the CF and non-CF mice. However, the levels of Muc3 (orthologue of human MUC17) were increased in the CF/Muc1-/- compared to the CF/MUC1.Tg animals. The amount of MUC1 mucin was increased several magnitudes in the CF mice, but did still not appear to be a major mucin. The amount of insoluble mucus of the large intestine was also increased in the CF mice, an effect that was partially restored in the CF/Muc1-/- mice. The thickness of the firmly adherent mucus layer of colon in the Muc1-/- mice was significantly lower than that of wt mice. The results suggest that MUC1 is not a major component in the accumulated mucus of CF mice and that MUC1 can influence the amount of other mucins in a still unknown way.