The 3-fucosyl N-acetyllactosamine residue is the antigen recognized by the monoclonal antibody MC2. Using MC2, we demonstrated the distribution of this antigen in a variety of squamous epithelia. The antigen is expressed to a variable degree on supra-basal cells in most normal non-keratinizing squamous mucosae, with a similar distribution in metaplastic squamous epithelia; antibody-labelled latex microspheres and immunogold electron microscopy show the antigen to form part of the glycocalyx. In dysplastic and neoplastic squamous lesions, expression is reduced or absent except in cells around areas of differentiation. Prior neuraminidase treatment of sections had little effect on the amount or distribution of demonstrable antigen. Expression of this antigen by cells in non-keratinizing squamous epithelia gives an indication of cell maturity and may provide a histological marker for the grading of dysplastic and malignant squamous mucosal lesions. A possible role for these carbohydrate residues in squamous mucosal defence is discussed.