Spleen cells derived from rats naturally-infected with Mycoplasma pulmonis were stimulated in vitro, and then fused with a mouse myeloma cell line. The resulting hybridomas were screened for mycoplasma-specific Mabs by ELISA and for hemolysis-blocking activities. Fusions performed with in vitro-treated spleen cells yielded larger numbers of growth-positive wells and antibody secreting cells than untreated spleen cells from the same animals. Hybridomas derived from naturally-infected animals gave a higher percentage of hemolysin-specific monoclonal antibodies than did hyperimmunized animals. This indicated that B cell priming during mucosal infections can produce antigen-primed spleen cells. Stimulation of these cells in vitro can result in monoclonal antibodies against antigens not normally recognized during immunization with in vitro grown pathogenic bacteria.