The capsular polysaccharide of Klebsiella pneumoniae (CPS-K) was shown to enhance markedly antibody response of mice to various antigens. The primary antibody response to bovine serum albumin (BSA) occurred more rapidly after an injection of BSA mixed with CPS-K than after injection of the same amount of BSA in Freund's complete adjuvant (CFA), although the peak antibody level in the serum after the primary injection became higher eventually in the latter mice. In secondary and tertiary responses, much more pronounced antibody response (occasionally more than 30 mg of antibody protein per ml) was induced in mice injected first with BSA mixed with CPS-K than in those injected originally with BSA in CFA. The adjuvant action of CPS-K was also much stronger than that of lipopolysaccharide of Escherichia coli and zymosan. CPS-K could induce the antibody response of mice to syngeneic eyeball extracts which are normally non-immunogenic.