In this review, we analyze data pertinent to the origins of specific immune recognition by lymphocytes. The phenomena of immunity in invertebrates and the cells that might be involved in these processes are considered. We conclude that the existence of vertebrate-type immunocompetent lymphocytes in invertebrates is not yet proven. All vertebrates apparently possess immunologically competent lymphocytes, and the following conclusions may be drawn: (1) Specific antigen recognition by lymphocytes of all vertebrates appears to be mediated by membrane-bound immunoglobulins. These immunoglobulins are, in all probability, distinct from secreted, serum immunoglobulins (antibodies), although serum and surface immunoglobulins share combining sites for antigen which are formed of variable regions. (2) There is evidence for similar functional divisions among the immune systems of all vertebrates, as reflected in the results of anatomical, functional, and physicochemical investigations of lymphocytes from animals as diverse as fish and mammals.