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Indirect evidence shows that the secondary structure of DNA in phage particles differs from that in solution [1][2][3]. There is [4] a substantial difference between the luminescence of acridine orange (AO) bound to one-strand DNA and of AO bound to two-strand DNA. AO can also interact with DNA within cerrain phage particles [5, 61. We have used this property of AO in direct study of the secondary structure of DNA in phage particles.We used the phages E. coli T2 and Klebsiella L1, which contain two-strand DNA, and also E. coil q)174, which contains one-strand DNA. The phage preparations were made by differential centrifugation [7], while the DNA was isolated by the phenol method [81. The phage particles were disrupted by heating to various temperatures in the range 25-98~The luminescence spectra (LS) were measured with an ISP-51 spectrograph fitted with an FEU-38 photomultiplier working into a narrow-band amplifier and phasesensitive detector. The exciting source was an SDV-120A lamp with a UFS-2 filter. The DNA concentration was 20-30 #g/ml, while the concentrations of the phage suspensions were deduced from the optical density at 260 nm. We used an aqueous solution of recrystallized AO. The luminescence measurements were made at room temperature.We measured the LS of the complexes of AO with native and denatured DNA from T2 and L1, and atso with DNA from q)X174. The DNA was denatured by heating to 100~ for 30 min, followed by rapid cooling