In vitro disassembly of tobacco mosaic virus (TMV) virions occurred in the presence of both polyornithine and a lipid fraction isolated from tobacco leaf membrane. The latter could be replaced by lecithine. Disassembly of 10 microgram of TMV virions was attained in the presence of a 500-mg leaf equivalent of membrane lipid and 20 microgram of polyornithine in 1 ml of 0.01 M Tris-HCl buffer, pH 7.4 at 30 C. Similarity and dissimilarity between the in vitro disassembly and the in vivo uncoating mechanisms are discussed.