We compared the primary response in vitro with the secondary response in vivo of A/J inbred and CD-1 outbred mice to digoxin-HSA. The frequencies of hybrid formation and growth, and of the hybridomas that secreted antibody to digoxin were similar in both strains regardless of the immunization procedure. The patterns of cross-reactivity of the monoclonal antibodies to two compounds structurally related to digoxin (ouabain and digitoxin) were likewise similar for both strains and immunization procedures.