Homologous recombination was used to generate a number of mutants of serogroup B Neisseria meningitidis B16B6 with the following characteristics: (i) an inability to bind human or porcine transferrin because of loss of both transferrin binding proteins (Tbp) A and B [strain B16B6(Str^r)/tbpAB] and (ii) an ability to bind porcine transferrin but not human transferrin [strain B16B6(Str^r)/tbpA^apB^ap] due to replacement of the meningococcal Tbp with the Tbp of Actinobacillus pleuropneumoniae. During construction of the B16B6(Str^r)/tbpA^apB^ap strain, transformants expressing only TbpA or TbpB of A. pleuropneumoniae were isolated [strains B16B6(Str^r)/tbpA^apB and B16B6(Str^r)/tbpAB^ap]. Expression of the A. pleuropneumoniae Tbp in N. meningitidis B16B6 was iron regulated and expressed under the control of the meningococcal promoter. The relative abilities of the meningococcal transformants to bind porcine transferrin were in the order B16B6(Str^r)/tbpA^apB^ap > B16B6(Str^r)/tbpA^apB > B16B6(Str^r)/tbpAB^ap. Of these transformants, only B16B6(Str^r)/tbpA^apB^ap could grow in the presence of porcine transferrin as the sole iron source, achieving a growth rate similar to that of the B16B6 parent strain in the presence of human transferrin.