The adjuvant effect of polyadenylic.polyuridylic acid [poly(A).poly(U)] on antibody production against recombinant hepatitis B surface antigen (rHBsAg) was investigated. Immunization of adult Balb/c mice with rHBsAg plus poly(A).poly(U) elicited significantly higher antibody responses than immunization of mice immunized with rHBsAg alone. Such an adjuvant effect was evident even in mice sensitized with subimmunogenic doses of the antigen.