Retinoids have been shown to inhibit the expression of the squamous differentiated phenotype and are effective inhibitors of the increase in transglutaminase activity, cholesterol sulfate, and sulfotransferase activity. This inhibition occurs at nanomolar concentrations. Moreover, specific structural requirements are critical for the activity of retinoids. The suppression of the squamous differentiated phenotype by retinoids might be mediated by nuclear retinoic acid receptors. This chapter discusses the assay of transglutaminase activity and assay of cholesterol sulfotransferase activity. The expression of transglutaminase types I can be analyzed using monoclonal antibodies and eDNA probes specific for the transglutaminases. Type I transglutaminase activity can be modulated by retinoids. Retinoids inhibit transglutaminase type I activity during squamous differentiation in several cell types, including epidermal keratinocytes and tracheobronchial epithelial cells. © 1990, Elsevier Inc. All rights reserved.