In a previous paper it has been shown that certain viruses adsorb strongly to cholesterol columns and that this system is useful for the study of virus-lipid interactions. The results of adsorption studies with a greater number of different viruses and a wide range of water-insoluble polar lipids (sterols, long-chain aliphatic compounds such as acids, alcohols, amides, and amines) are reported. Data on the lipophilic affinities of different viruses, on the adsorption capacities of different lipids, and on infectivity of lipid-virus suspensions permit the following conclusions: (1) Viruses can be classified into lipophilic (influenza, NDV, vaccinia, herpes simplex, infectious canine hepatitis) and hydrophilic viruses (polio, Coxsackie, ECHO), according to their ability to interact with lipids. (2) Lipophilic viruses can be subdivided according to their differential adsorption affinities for lipids of increasing adsorption activity. (3) The strength of the bond, i.e., the reversibility of the adsorption, can be measured by determining the adsorption capacity of different lipids for a given virus and by titrating the infectivity of the corresponding lipid-virus suspensions. Of the lipids tested, hexadecylamine (HDA) is by far the most active adsorbent, capable of binding as much as 50,000 hemagglutinating (HA) units of influenza virus per milligram. In contrast to the other lipids, it combines irreversibly with virus as evident from the loss of infectivity of HDA-virus suspensions. The high adsorption capacity and binding strength of HDA for influenza virus parallels the strong adjuvant effect produced in guinea pigs immunized with HDA-adsorbed influenza vaccine.