We have developed a very rapid method for the detection of the Philadelphia chromosome based on the polymerase chain reaction. This chromosomal translocation generates bcr/abl fusion mRNAs, which were transcribed in cDNAs and subsequently amplified with two sets of primers. Through the use of an additional internal primer set the method reaches the sensitivity to detect a single Ph1-positive cell among 10(5) unaffected cells without any blotting procedures. The whole analytical procedure starting from RNA isolation up to agarose gel electrophoresis can be performed in about 5 h. First results indicate that the frequency of the Ph1 translocation exceeds 40 per cent in certain immunologic subtypes of adult acute leukemia.