We have developed a rapid method for the detection of bcr/abl mRNAs, the products of the BCR/ABL fusion genes. The method is based on the polymerase-chain-reaction (PCR). Through the use of additional internal primers it is possible to detect directly a single Ph1-positive cell among 10(5) unaffected cells thus omitting time-consuming blotting procedures. The whole analytical procedure starting from RNA isolation to agarose gel electrophoresis including two rounds of PCR can be performed in less than six hours.