The antiviral extract from Rhodopseudomonas capsulata was purified and the predominant active component was defined as cis-vaccenic acid (Cl-8:1 delta 11) by gas-liquid chromatography and gas chromatography-mass spectrometry analyses. Antiviral activities of unsaturated fatty acids and related alcohols against T5 phage were also tested. Among them, linoelaidic acid, oleic acid, and petroselenyl alcohol inactivated 98%, 53%, 67% of T5 phage at the concentration of 50 micrograms/ml, respectively.