Polyamines were measured in viruses using different techniques. An improved method of polyamine analysis is reported for tobacco mosaic virus (TMV), a rod-shaped virus (95% protein and 5% RNA), based on HPLC of sonicated PCA-treated highly purified virus suspensions. This method allowed higher and more reliable recovery of TMV polyamines (putrescine, spermidine and spermine) when compared to the HPLC of non-sonicated samples and to thin layer chromatography. It is suggested that sonication acts on PCA-precipitated protein aggregates causing the release of trapped polyamine molecules.