Using the technique of in situ hybridization, we have attempted to identify messenger RNA for tumour necrosis factor-alpha (TNFα) in cells infiltrating allergen-induced late phase reaetion (LPR) of the skin and the nose ofatopic subjects. We have also compared the number of TNFα mRNA positive cells in bronchoalveolar lavage (BAL) from atopic asthmatics and normal controls. Twenty-four hours after local allergen challenge, 12/14 skin biopsies and 9/10 nasal biopsies had positive hybridization signals for TNFα mRNA whereas only 4/14 and 2/10 biopsies were positive in the relevant diluent controls. Compared with diluent sites significantly increased numbers of cells expressing mRNA for TNFα were observed in the LPR of skin (P <0.004) and nose(P <0.006). All BAL from asthmatics (n= 10) and from normal volunteers (n= 10) had cells showing positive hybridization signals for TNFα mRN A but these were at increased frequency in asthmatics (P< 0.001). These results suggest that TNFα may be an important cytokine in atopic allergic inflammation.