The number of EcoRI restriction sites in DNAs of several lombdoid phages varies from 2 to 7 and the size of fragments ranges from 0.6 . 10(6) to 25 . 10(6) daltons. Effectiveness of restriction determined in vivo weakly correlates with a number of restriction sites, suggesting different sensitivity of different sites to EcoRI action. No correlation between molecular weights of DNA of different phages and the rate of heat inactivation of the phage has been found. The variability of number of the restriction sites and of the molecular weights of the fragments permits to conclude that: 1) closely-related organisms can be possibly used as convenient sources of certain genes (if a construction of new vector lambdoid phages is essential); 2) there is an opportunity to find phages with different distribution of the restriction sites to other restrictases within a group of phages with non-identical EcoRI restriction sites.