I understand my role in this conference to be one of explaining why I think it is essential that scientists continue to have reasonably easy access to human cells and tissues for use in research. My view is that of a virologist who works in the area of human diseases caused by viruses, one who daily uses a variety of human cells and tissues. I believe the use of such tissues to be so important and necessary that I have some difficulty in understanding how anyone could think otherwise. Assuming that most people think it important to curb disease, my natural tendency is to feel that if there are people who would consider placing major restrictions on the use of human cells and tissues in such areas as virus research, it must be because they do not have the appropriate information available to them. It seems to me that if one knows even rudimentary things about viruses, the necessity for cells and tissues in research becomes clear. I propose, therefore, to tell you a few things about viruses and the problems which are faced by virologists in the expectation that you will then see the need. It is difficult to provide a simple definition of a virus. Most commonly today we say that a virus outside a cell, i.e. in a test tube, consists of genetic material enclosed in a protective coat of protein. In cells, however, most of the protein is unnecessary and there the virus is a package of genetic information, i.e. a set of genes, with the potential for multiplication and for transfer to another cell. Viruses have no metabolism of their own, no motility, and they do not multiply and grow in the manner of other microorganisms. Viruses multiply only within living cells and they do so by making their genes functional parts of the cell and thereby using the synthetic apparatus of the cell to produce more virus. The cells of which I am speaking may be ones in the tissues of an animal or person or they can be ones removed and cultivated in tubes or other containers. For certain viruses they can be cells of bacteria, plants or insects; but cells are the natural habitat of viruses to the degree that it has appeared reasonable to many virologists that viruses have been derived by some unknown mechanism from the genes of cells. Since viruses which we find outside cells are relatively inert particles in a transport phase-on their way from one cell to another-the virologist is quite dependent upon cells. Living, viable cells are required for production of quantities of a virus and for study of the processes of viral multiplication. Furthermore, we are dependent upon cells for measurements of numbers of virus particles and for recognition and identification of viruses. We can recognize certain chemical and structural characteristics which are virus-like, but it is the effect of viruses upon cells that is of utmost importance. Multiplication of some viruses in cells causes death of the cells, but cell death is not the only possible effect. The presence of viral genes in a cell may alter cell function or cell behavior 662