Using non-radioactive in situ hybridization we studied the expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and PolyA+ RNA in psoriasis and normal skin, and GAPDH in epidermis following application of a range of noxious stimuli, including ultraviolet radiation, Sellotape stripping and various irritants. In keeping with what might have been expected from previous results on cell culture of various non-epidermal cell types, expression of these putative control gene products is not constant. We suggest that the use of GAPDH, and possibly other control genes, may lead to error in interpreting experiments using Northern blotting or even RT-PCR of epidermal samples.