Monoclonal antibodies (MAbs) were complexed to solid matrices and used to purify virus proteins from simian virus 5 (SV5)-infected tissue culture cells, ideally in such a way as to bind equimolar amounts of antigen to antibody. The resulting solid matrix-antibody-antigen (SMAA) complexes were then used as immunogens and successfully induced specific humoral and cytotoxic T cell responses. By attaching more than one MAb to the solid matrix and using such complexes to purify the respective proteins multivalent immunization was achieved. Analysis of the cytotoxic T cell response of immunized animals indicated that both surface and internal SV5 structural proteins can act as target antigens. Immunization with SMAA complexes, in the absence of adjuvant, induced higher levels of antibody than the antigen alone precipitated on alum. However, immunization with SMAA complexes resulted in relatively less antibody being produced to the antigenic determinant through which the protein is coupled, via antibody, to the SMAA complex compared with the amount of antibody produced against other antigenic determinants on that protein. The particulate solid matrix used to form the SMAA complexes in most of the experiments was a 'fixed' and killed suspension of the Cowan A strain of Staphylococcus aureus, although preliminary results indicated that Protein A-Sepharose could also be used as a solid matrix. Prior immunization with S. aureus alone did not reduce the level of the immune response to the appropriate antigen on subsequent immunization with S. aureus-antibody-antigen complexes. In fact on immunization of mice with these complexes the level of antibody produced to the S. aureus matrix itself was less when S. aureus-antibody-antigen complexes were used as immunogens than when S. aureus or S. aureus-antibody complexes were used. Furthermore, rabbits immunized with S. aureus-mouse MAb-antigen complexes showed a vigorous immune response to the antigen.