Identification of DNA mutations in urine sediments has been proposed as a noninvasive and early indicator of urinary tract cancer (1)(2)(3)(4). The sensitivity of this approach is limited by the proportion of DNA containing the mutant allele, because the resolution of the technique used to detect oncogenes or suppressor gene mutations (mainly single-strand conformational polymorphism) requires ∼10% representation of mutant product (4)(5), whereas estimates to the proportion of tumor cells in the urine of bladder cancer patients were not >7% (1). H- ras represents a single member of a family of genes coding for a 21-kDa protein involved in the regulation of cell growth and differentiation (6). Most activation events of this protooncogene to the oncogenic form found in human bladder cancer occur via a point mutation at codon 12 (7)(8). To provide a more sensitive means for detecting mutations, we have devised a sensitive, nonradioactive procedure based on two consecutive PCRs with intermediate restriction digestion, which leads to effective enrichment of mutant alleles for further amplification and elimination of wild-type alleles by digestion (5)(9)(10)(11). Wild-type DNA was prepared from the urine sediments of three healthy volunteers, and two DNA samples containing known mutations in H- ras codon 12 were mixed with an equal quantity (200 ng) of that wild-type DNA. These mutant-type samples were extracted from tumor specimens after surgical resection. The mutations were known through direct sequencing. Amplifications in the first round were carried out in a final volume of 100 μL in 1× PCR buffer containing 2 mmol/L MgCl2, 50 pmol of each primer, dNTPs at 40 μmol/L each, and 2.5 units of UlTma® DNA Polymerase (Perkin–Elmer–Cetus). After an initial denaturation step at 94 °C for 2 …