For 59 different viruses, when the amount of nucleic acid in the particle is related either to the dry weight of the particle or to the particle volume, two classes of virus groups emerge--those with enveloped or those with geometrical particles. The enveloped viruses have particles with the following properties: (i) about 40 X 10-6 daltons of anhydrous weight per 10-6 daltons of nucleic acid; (ii) a particle volume of about 2 X 10-5 nm3 per 10-6 daltons of nucleic acid; (iii) a limiting lipoprotein membrane. These properties are qualitatively and quantitatively close to those of prokaryotic cells. The geometric viruses have particles with roughly one-tenth the anhydrous mass per unit of nucleic acid and one twenty-fifth the particle volume per unit of nucleic acid. They do not possess a limiting lipoprotein membrane.