The chemical accessibility of tyrosyl residues in TYMV capsids was studied by spectrophotometric titration and with the nitrating agent tetranitromethane. That of the lysyl residues was probed with trinitrobenzenesulfonate. Attempts to test their accessibility in virions were also made. Since some of these reactions were accompanied by structural changes, degradation of the particles were monitored with ultracentrifugation and light-scattering measurements. Alkaline titration of TYMV capsids induced ionization of two of the three tyrosyl residues per subunit at pH 11.3, but the third tyrosyl ionized with an apparent pK of 12.65, concomitantly with the degradation of the capsids. Reaction with tetranitromethane suggested that one tyrosyl residue per subunit can easily be nitrated and initiates degradation, after which the remaining residues also react. In intact capsids, five out of seven lysyl residues per subunit reacted readily with trinitrobenzenesulfonate. The other two lysyl residues were trinitrophenylated only after degradation of the capsids. On the other hand, all seven lysyl residues per subunit were easily trinitrophenylated in virions, during which reaction the virions disintegrated. The demonstrated chemical inaccessibility of specific numbers of tyrosyl and lysyl residues in TYMV capsids and the observed structural consequences to the capsids when the residues were made to react are consistent with previously published properties of the cysteinyl and tryptophanyl residues. The findings suggest that in the capsid the central region of the TYMV polypeptide chain is buried and might represent a site of contact between neighboring subunits.