The adjuvant action of the 03 antigen of Klebsiella (K03) on the antibody response to sheep red blood cells (SRBC) was elucidated by injecting both K03 and SRBC subcutaneously at the right inguinal region of SMA mice. We demonstrated that K03 exhibits a novel ability to augment anti-SRBC plaqueforming cell responses in both the local lymph node and the spleen at a relatively late stage of immunization. Escherichia coli lipopolysaccharide, dextran sulfate and concanavalin A showed such an action only minimally. In parallel with the development of the adjuvant action, K03 definitely activated B cells in the local lymph node polyclonally for either IgM or IgG synthesis, suggesting that the mechanism of the adjuvant action includes direct stimulation of B cells by K03 at the late stage. Neither increase in trapping of lymphocytes in the local lymph node nor change in tissue distribution of antigen was shown to be primarily involved in the mechanism of the adjuvant action.We have reported that the polysaccharide extracted from the culture supernatant of Klebsiella pneumoniae type 1 strain Kasuya (03:Kl), formerly designated CPS-K or neutral 21) and recently shown to be serologically identical to the 03 antigen of Klebsiella (K03) (14), exhibits a strong adjuvant action on antibody responses (17,18,21) and on induction of delayed-type hypersensitivity (30) to soluble protein antigens including saline extracts of syngeneic tissues (17,20) when injected subcutaneously (sc) together with the antigens. This action of K03 was much stronger than that of Escherichia coli lipopolysaccharide (LPS), dextran sulfate (DS), polyadenylic-uridylic acid, concanavalin A (Con A), and Freund's complete adjuvant (17,28). On the other hand, K03 did not augment antibody responses more strongly than did E. coli LPS when injected intravenously (iv) to-843