AbstractUsing the well‐differentiated human squamous carcinoma cell line LICR‐LON‐HN‐5 as an immunogen we have produced a monoclonal antibody (32a) that reacts with the keratohyalin granular layer of the normal epidermis. We present here results showing the distribution of the epitope recognized by this antibody in human tissuesin vivoandin vitro, as demonstrated using immuno cytochemical staining techniques at the light and ultrastructural levels. Expression of the determinant first appears at 18 weeks of fetal development, localized in cells associated with the hair germ. In hyperplastic epidermis the staining pattern is altered, apparently linked with a switch from orthokeratotic to parakeratotic keratinization. In primary squamous cell carcinomas and in xenografts formed by the squamous carcinoma cell line LICR‐LON‐HN‐5 the keratinized elements are stained. Poorly differentiated tumours are not stained, indicating that antibody may be useful as a marker of terminal differentiationin vivo. When grown on collagen gels both human epidermal keratinocytes and the squamous carcinoma cell line show staining of the more differentiated cells which appears to be associated with the keratinohyalin granules, indicating that this antibody may be of value in studies aimed at the control of squamous di