AFTER exposure of animals to most antigens, formation of antibody by cells of the B lymphocyte lineage requires helper effects of T lymphocytes1. The degree of thymus dependence varies for different antigens, and slowly metabolised immunogens with repeating epitopes, generally polysaccharides or polymerised forms of protein, seem to be the most thymus-independent2. Antigens of some viruses, however, may be relatively thymus independent. Inherent in the structures of many viruses is a rigid geometrical array of repeating antigenic determinants which, if the spatial relationships were appropriate, may effectively interact with and trigger B cells without help from T cells. Also, glycoproteins are important antigens in the virions of many viruses3, and the carbohydrate portions of these molecules may function as thymus-independent immunogens. In addition, repeated immunisation of mice depleted of T cells can sometimes result in antibody production to thymus-dependent antigens4, and virus infections often result in the sustained production of antigens and thus prolonged exposure of the infected host to the antigens. These antigens may be presented to the immune system in a variety of ways—on the membranes of infected cells, as soluble antigens released from such cells, and as repeating epitopes on virus particles.