Monoclonal antibody (mAb) SV5-P-k was obtained following immunization of mice with purified preparations of the LN human isolate of simian virus 5 (SV5) (Randall et al. 1987). It was the only antibody isolated in this process, which could distinguish between two dog isolates of SV5 differing in their abilities to establish persistent infection (Southern et al. 1991). The antibody was specific for an epitope in the SV5 phospho (P) protein, in which leucineto-proline substitution at position 102 abolished the binding activity. The binding of SV5-P-k to the native P protein could be successfully competed in a radioimmunoassay by a synthetic nonapeptide, localizing the recognized epitope (termed Pk) within the sequence 98-IPNPLLGLD-106. It was demonstrated later that the Pk epitope, extended by 3 and 2 amino acid residues derived from the SV5 P protein sequence at the Nand C-termini, retained its antigenicity when coupled to the C-terminus of a larger protein (Hanke et al. 1992). This observation has been extended for attachment to both Nand C-termini of over 20 foreign proteins, some of which were transmembrane glycoproteins (Hanke et al. 1994; Hanke et al. 1995; M. O'Reilly, unpublished results). Moreover, the strong affinity of the mAb SV5-P-k for the 14-amino acid tag, termed Pk, proved to be useful for the construction of solid matrix-antibody-antigen (SMAA) complexes, in which the antibody facilitated purification and potentiated immunogenicity of purified Pk-tagged antigens. SMAA complexes, when employed as multi-subunit vaccines, can induce antigen-specific humoral and cell-mediated immune responses, including cytotoxic T lymphocytes (CTL), in both mice and non-human primates (Hanke and co-workers, unpublished data; Randall and Young 1991). Thus, in A