Mucins are large glycoproteins protecting mucosal surfaces throughout the body. Their expressions are tissue‐specific, but in disease states such as cystic fibrosis, inflammation and cancer, this specificity can be disturbed. MUC5AC is normally expressed in the mucous cells of the epithelia lining the stomach and the trachea, where it constitutes a major component of the gastric and respiratory mucus. A number of mAbs have been raised against the gastric M1 antigen, an early marker for colonic carcinogenesis. Several of these mAbs recognize epitopes present on MUC5AC, suggesting that MUC5AC is the antigen. However, some of the mAbs raised against the gastric M1 antigen are widely used as antibodies against MUC5AC, despite the fact that their specificity for MUC5AC has not been clearly shown. In this study, we have tested the reactivity of the latter antibodies against a recombinantly expressed C‐terminal cysteine‐rich part of human MUC5AC. We demonstrate for the first time that the widely used mAb 45M1, as well as 2‐12M1 and 166M1, are true antibodies against MUC5AC, with epitopes located in the C‐terminal cysteine‐rich part of the mucin.