Abstract: Disclosed is a method to detect clinically relevant mutations of the DNA sequence of the KI-ras oncogene in stool DNA, its use and a testkit based thereon for early diagnosis of tumors, especially tumors of the pancreas and the colon. According to the invention, the method of detection is distinguished by extraction of genomic DNA from stool samples in a series of cleaning operations designed to eliminate inhibitor substances, and by base-complementary hybridization reaction by adding six different oligonucleotides with a defined complementarity to the clinically relevant mutated sequence fragments of the KI-ras gene.