To evaluate the 5' spliced form of human cathepsin B mRNA in colorectal mucosa and tumors, we have determined the ratio of the spliced form for the exon 2/the complete form of cathepsin B mRNAs obtained by RT-PCR. Such ratio is significantly higher in colorectal tumors than in colorectal mucosa (p < 0.05, Kruskal-Wallis test) or in skeletal muscle (p < 0.05). Moreover, 2-fold more complete form than the spliced mRNA was found in the tumors than in the colorectal mucosa. Our data indicate that the alternative splicing of human cathepsin B mRNA in the 5'UTR may be considered as an indicator of the cellular transformation, in colorectal cancer.