The induction of in vitro primary and secondary humoral immune responses to influenza virus in murine splenic explant culture is described. Anti-influenza antibody synthesized in vitro was detected and quantitated by a radioimmunoassay which utilized influenza coupled to bromoacetylcellulose. Both in vitro primary and secondary responses could be stimulated over a large range of virus doses. In vitro secondary responses were maximal when spleen donors had been immunized by the parenteral route although secondary type responses could be demonstrated as well after primary immunization by pulmonary infection. In vitro stimulation with influenza virus was relatively insensitive to inhibition at high antigen doses. The results are discussed in terms of the response to other antigens in spleen fragment culture.