The polyamine spermidine and the diamine putrescine have been detected in coliphages T5 and phiX174. Polyamines were identified by thin-layer chromatography and mass-spectrometry of dansyl derivatives, as well as by ion-exchange chromatography. In phiX174 phages, polyamines were sufficient to neutralize 0.5% of DNA phosphates. The polyamine content of T5 phages depended on growth media and purification procedures, but at least 1% of DNA phosphates were neutralized by polyamines. After infection, an increase in cellular polyamine was noticed. This increase paralleled variations in ornithine decarboxylase activity.