A method for the in vitro immunization of human tonsilar lymphocytes is described. This method allows for the sensitization of B-cells against various antigens under serum-free medium conditions. The medium is supplemented with defined concentrations of antigen, rIL-2 and muramyl dipeptide to boost the immune responses. This method may prove useful for the generation of antigen reactive B-cells for use in human monoclonal antibody production and the study of the effects of other lymphokines on the in vitro immune response.