Combination between immunogen and antibody molecules present on the relevant antigen-reactive cell(s) is commonly considered an early, essential step in the induction of detectable, high-rate antibody formation. Lymphocytes have been shown to display immunoglobulin-like molecules on their outer cell surface (1-3) and various methods have demonstrated a selective adsorption of antigen to the surface of a minority of the lymphoidal cell population (4-8). The increase of the number of reactive cells in such systems after immunization (4-10) and the specific inhibition of the reactions by blocking antigen (7,8) indicate that lymphoidal cells can display antibody activity on their surfaces. Direct demonstration that these surface receptors could be linked to potential immune capacity of the cells carrying the surface antibodies has come from experiments where filtration of immunologically competent cells through antigen-coated columns was carried out (11,12). In such experiments, specific elimination of the antigen-reactive cells directed against the antigen used for coating the column was obtained as indicated by the reactivity of the passed cells.Since we believe that preformed cell-bound antigen-specific receptor molecules are of prime importance during the induction of immune processes, we have tried to obtain further information by using antigen-coated columns. We have applied the technique to nonimmunized lymphocytes. Emphasis was put on analysis of the size of the antigen-combining site of the receptors by the use of columns coated with different hapten-carrier complexes. Furthermore, investigations were made to explore whether cells potentially capable of producing antibodies of different immunoglobulin classes can be separated in a similar manner. Evidence for the existence of preformed receptors on potential antibody-forming cells directed against complex antigens, as well as against chemically well defined substances of haptenic nature will be presented. The theoretical implications of the findings will be discussed.