The division of the lymphoid system into two major functionally distinct kinds of cells has increased immensely understanding not only of normal immunity but also of neoplasia affecting these cells. It is now abundantly clear that certain lymphocytes are conditioned by the thymus (hence their designation as T-cells) and acquire the ability to mediate delayed hypersensitivity and the rejection of foreign transplanted tissue. T-lymphocytes have a major role in the body's defense against viruses, fungi and certain bacteria such as tubercle and brucella bacilli. They elaborate such lymphokines as chemotactic factors, migration inhibition factor and a mitogenic factor. Thymus-independent lymphocytes, . . .