Using digoxigenin-UTP-labelled human HAM-1 (92 bp) or SMUC41 (850 bp) cRNA probes, the expression and localization of MUC2 gene transcripts were determined by in situ hybridization in human nasal tissues obtained as biopsies from 12 patients with cystic fibrosis (CF): all had been part of a gene therapy trial in which CFTR cDNA-liposome complexes had been delivered by topical application to eight and liposome alone to four as a placebo control. For comparison, there were nasal tissues taken at surgical resection from four non-CF subjects and a further four biopsies taken from normal healthy volunteer controls. Both SMUC41 and HAM-1 probes provided a strong signal. MUC2 mRNA transcripts were present in serous and mucous acini of submucosal glands, ciliated and basal cells of the surface epithelium, and occasional mononuclear inflammatory cells. The percentages (mean +/- SEM) of serous and mucous acini showing positivity for MUC2 gene expression in the four samples surgically resected from non-CF subjects were 25.4 +/- 5.6 and 26.7 +/- 3.3 per cent, respectively. Compared with the non-CF subjects, the mean percentage of acini showing MUC2 gene expression in the four placebo-treated CF subjects was significantly higher for serous (80.5 +/- 12.7 per cent; P < 0.05, t-test), but not for mucous acini (53.1 +/- 16.8 per cent; P = 0.38). In CF and non-CF groups, where present, MUC2 positivity was strongly expressed and constituted approximately 84 per cent of the cell area in serous acini, whereas it was less obvious and was confined to the perinuclear area of cells in mucous acini. A significantly greater proportion of the surface epithelium was positive for MUC2 mRNA transcripts in the CF subjects (89.0, +/- 1.4 per cent) than in the surgically resected tissues of the four non-CF subjects (19.4, +/- 4.0 per cent) (P = 0.02). In the eight CFTR-cDNA-treated subjects, there was an overall trend to reduction, but no statistically significant alteration of MUC2 gene expression. It is concluded that the MUC2 gene is expressed at three- to four-fold higher levels in CF nasal mucosa than in non-CF nasal tissue and that it is expressed in a variety of cells additional to submucosal mucus-secreting glands.