Publisher Summary This chapter reviews the structure, function, and immunological properties of C-type virus-directed macromolecules that are immunologically active, and therefore approachable by immunological techniques. Antigen is used to designate such molecules, essentially because the chemical constitution of many of them is not yet known. The mechanism of virus assembly by a budding process suggests that the virus may contain many host cell constituents that are not essential to the virus structure, but rather represent virus-associated “contaminant.” It also seems feasible that as a result of virus-induced changes in cell physiology new antigenic determinants appear either by the exposure of hidden antigenic macromolecules or through the synthesis of new antigens as a result of the derepression of cellular genes that are inactive in a normal cell. The chapter focuses on C-type viruses of different species, which are closely related with respect to fundamental properties. They mature by budding from the cell membrane, are similar in the morphology, size, and constitution of nucleic acid, proteins, lipids, and enzymes, and require similar cell growth conditions for infection.