SUMMARYThe sedimentation coefficients of tobacco necrosis virus strain D (TNV•) in sodium phosphate at pH 5 and pH 7 are io4S and 97S respectively. The change in sedimentation coefficient between pH 5 and 7 is fully reversible and the decrease on transfer to pH 7 is unaffected by the presence of Mg 2+. TNVD is disrupted byBacillus subtilis protease at pH 7"5 but not at pH 5; ribonuclease neither disrupts virions nor destroys the infectivity of TNVD at either pH 7"5 or pH 5. Virions are markedly unstable at alkaline pH but the effectiveness with which different buffers dissociate TNVD varies. Virus is largely resistant to dissociation by I M-NaC1, i M-urea, io M-ethanediol and I% sodium dodecyl sulphate (SDS) at or below pH 6 and is largely susceptible to dissociation by these agents at or above pH 7. Our results suggest that as the pH is lowered some non-covalent bonds important for virion stability may be formed and/or some electronic repellent interactions causing virion instability may be weakened.