BMV development was studied in well defmed physiological conditions, avoiding the use of antibiotics or excision of the roots. For an adequate labelling, to distinguish between host and viral RNAs, different periods of exposure to 32p were required, depending upon the post-inocu1ation time of infected plants and corresponding age of healthy controL Within the period of normal outlook of the plants, the rate of 32p· incorporation into BMV RNA was much higher than into barley n'boso­mal RNA in infected tissue or in comparable healthy tissue.Analysis of the replicative structures isolated from infected plants revealed that only the three largest RNAs (A,K,B) had their own replicative intermediates (R I). This material was shown to con­tain a certain amount of intact viral RNA species, which were stable when treated with heat and for­mamide, excluding the possibility of hidden breaks.