The assembly of viruses will be studied to elucidate the molecular mechanisms involved in the morphogenesis of biological structures. We shall approach this pboblem in the following ways: (1) We shall continue our investigations of the components and functions of the host cell which are required for viral morphogenesis. By the isolation and study of new types of host defective cellular mutants, we shall learn more about the roles the host cell plays in virus production. Further information regarding virus-host cell interactions may be obtained by studying the viral mutants which can overcome the various host defective cellular mutants. (2) It seems clear from the discovery of host defective cellular mutants that a cell may be altered so that it remains viable but lacks the ability to produce certain viruses. We hope to discover compounds which cause analogous nonlethal alterations in cells so that treated cells would be viable, but would be unable to replicate particular viruses. In preliminary studies we have found some reagents which appear to effect such cellular alterations. (3) The association of viral proteins with host cell membranes is a widespread phenomenon among animals and bacterial viruses. Using conditionally lethal viral mutants and host defective cellular mutants we shall isolate membranes to which specific viral proteins are bound. We shall perform experiments to determine the types of physical and chemical bonds by which these viral proteins are linked to the host cell membrane. (4) The capsids of some viruses appear to assemble, and then to be filled with nucleic acid. The study of conditionally lethal viral mutants and host defective cellular mutants which cause the accumulation of empty virus particles may enable us to gain information related to the mechanisms by which nucleic acid molecules are packaged into preassembled viral capsids. The information obtained by these studies should lead to rational methods by which productive viral infections can be blocked.