Immunogenic, antigenic and adjuvant properties of a polyanionic copolymer of styrene and maleic anhydride (PSM) are described. The results obtained showed that PSM was immunogenic in BALB/c mice when administered without adjuvant, the optimal dose being 0.01 mug/mouse. Antibodies could be detected only by precipitation in gel but not by agglutination. High molecular weight polymer (290 000 daltons) was a stronger immunogen and antigen than copolymers of lower molecular weights (85 000-210 000 daltons). PSM also showed adjuvant activity and enhanced the humoral response of mice against sheep red blood cells. Adjuvanticity was found only when 1 mug of PSM/mouse was applied. The results obtained suggest that PSM interacts with B rather than T cells.