SUMMARY

The multiplication and inactivation of tomato bushy stunt virus (TBSV) and of bromcgrass mosaic virus (BGMV), both of which have sphcrical particles, differed greatly at different temperatures. TBSV was inactivated in and disappcarcd from infected plants at 36°; BGMV maintained its contcnt and virus produced at 360 was as infective as that produccd at 2o °. Infectivity of TBSV was lost in vivo or in vitro without any apparent changc in the physical properties of the particles or of the nuclcic acid. BGMV was unaffccted by heating in vitro for an hour at 45 ° at pH 5"8, but at pH 7"o lost its infcctivity within an hour at 36 °. Even at 20 ° the particles wcrc unstable and, in the presence of pancreatic ribonuclcase, some but not all particles disintegrated to producc two new components. BGMV from plants infected for I wcek was, wcight for wcight, more infcctivc than virus from plants infected for 3 weeks, and contained proportionally more 27 S nuclcic acid than non-infective 22S or 14 S, RNAs. Whcn virus was heated at 3 I ° in buffer pH 6.8 the amounts of $27 and $22 nucleic acid diminishcd, but amounts of 14S nucleic acid diminishcd only at higher tempcraturcs. Thus inactivation of BGMV, but not of TBSV, may rcflcct breaking of the nucleic acid