The frequency of active ras oncogenes in human bladder cancers associated with schistosomiasis, the cause of which is suspected to be a chemical carcinogen(s) in urine, was examined. Of 9 squamous cell carcinomas of the bladder surgically obtained in Egypt, none scored as positive in the regular DNA transfection assay using NIH/3T3 cells as recipients. The restriction fragment length polymorphism assay at codon 12 of the H-ras gene confirmed the absence of an activating mutation at this site in all of them. Western blotting analysis of electrophoretic mobilities of the ras p21 proteins, a method which can detect at least some of the point mutations within codons 12 and 61 of ras genes, suggested a point mutation within codon 61 in one out of the 7 tumors analyzed. In contrast to the low frequency of detection of mutationally activated ras oncogenes, enhanced expression of the ras p21 proteins was demonstrated in 4 of them by this analysis. The carcinogenic process involved in the endemic bilharzial bladder cancers is thus not associated with detectable point mutations within ras genes at a higher frequency than those in non-bilharzial bladder cancers in Japan or the USA.