A method based on RNA-transcript conformation polymorphism (TCP) was tested for detection of two PVY isolates in a mixed infection. Differences in electrophoretic mobility of RNA transcripts copied from PCR products of each virus isolate enabled the distinction between the two virus isolates in a mixed infection. The identities of the RNA transcripts and hence of the infecting virus isolates were determined by annealing to reference oligonucleotides containing unique strain-specific sequences visualized by retardation of transcript mobility in gel. The ratio at which both virus isolates could be detected was as low as 1:10. The suitability of this procedure for the study of mixed virus infections is discussed.