DNA sequences homologous to the known viral oncogenes (v-onc) of certain retroviruses have been found in a variety of normal uninfected cells, including those of man. These genestermed cellular oncogenes (c-onc) appear to have been the evolutionary progenitors of the v-onc genes. It is suggested that cellular oncogenes may possess an oncogenic potential because of the structural similarity between c-onc genes and their viral homologues. One line of evidence supporting this idea is that transforming genes in human tumor cell lines and tumors have been identified as the c-onc genes known as c-ras^(Ha) and c-ras^(Ki). Activation of cellular oncogene found in human urinary bladder carcinoma seems to be activated by point mutation. Furthermore, considering the multistep carcinogenesis in human tumor in vitro, it seems to be very important to the expression level of multiple c-onc genes in freshly obtained human tumors. We, therefore, investigated the level of gene expression of five c-onc genes (c-ras^(Ha), c-ras^(Ki), c-myb, c-myc and c-fps) in 6 transitional cell carcinomas of human urinary bladder, using the presence of messenger RNA(mRNA) transcripts as a parameter of gene expression. DNA probes used in DNA-RNA hybridization technique were all cellular oncogenes except ras^(Ki) gene (v-ras^(Ki)). More than one cellular oncogenes are transcriptionally active in all of tumors examined and transcriptional activity of certain oncogenes was greater in the malignant than the normal tissue.