Complex formation between synthetic polyelectrolytes (PE) [poly-4-vinyl-N-ethyl (cetyl), pyridine bromides-PVP(R2, R16)], bovine serum albumin (BSA), or 17 beta-estradiol-BSA conjugate (BSA.E) was studied in neutral water. Weakly water-soluble (colloidal) complex was formed upon addition of BSA.E to PVP (R2, R16) solution at pH 7. A nonrandom distribution of the protein molecules between the coils of polycations and self-assembly in the nonstochiometric polycomplex particles took place. The immunogenic properties of PVP (R2, R16)-BSA.E polycomplex were investigated and the specificities of produced antibodies analyzed. 17 beta-Estradiol introduced in polyelectrolyte complexes (PE-BSA) was found to invoke considerable increases in the steroid-specific immunoresponse. However, a comparative study of immunogenic activity of polycomplexes versus BSA.E+incomplete Freund's adjuvant (IFA) mixtures revealed some differences in regards to the specificity of antibody production. In contrast to IFA+BSA.E systems, polycomplexes were able to generate estradiol-as well as BSA-specific antibodies. Such a carrier-directed response may be determined by increase in immunogenicity of weak antigenic determinants and/or by the exposure of internally located determinants upon complex formation with polyelectrolytes. Fusions following the two different immunization procedures resulted in the growth of comparable numbers of estradiol-specific monoclonal antibodies with apparently similar antigen affinities. Thus, immunizations using antigens in PEC appear to provide an efficient alternative to IFA.