Mucins (MUCs) are high molecular weight membrane glycoproteins. The gene expression of MUCs (MUC1-MUC8) may change characteristically during malignant transformation of epithelial tissues. Total RNA was isolated from the four bladder cancer cell lines RT4, 647V, HT1376, and 486P (pathological gradings between G1 and G4) and 17 samples of transitional cell carcinomas, as well as 16 samples of normal human urothelium of the bladder from surgically removed specimens. The RNA samples were studied with MUC1-, MUC2- and MUC7-specific nested reverse transcription-PCRs. Gene expression of MUC1 and MUC2 was found positive in all normal, as well as in malignant, tissue samples and in the tumor cell lines. In contrast, gene expression of MUC7 was only detected in bladder cancer cell lines and samples of invasive transitional cell carcinomas, but neither in superficial, noninvasive bladder tumors nor normal bladder urothelium. Only one of the samples of normal urothelium obtained from 16 different tumor-bearing bladders was positive for MUC7 gene expression. These results suggest a differential MUC7 gene expression with the onset of malignant transformation of the bladder urothelium.