The experiments presented here were performed to evaluate immune responsiveness of hepatitis B virus (HBV)-transgenic mice (transgenic mice), as a model of HBV-carrier state to a T-cell-dependent antigen, keyhole limpet haemocyanin (KLH). The transgenic mice which were completely unresponsive to hepatitis B surface antigen (HBsAg), responded poorly to KLH. The levels of anti-KLH antibodies (Ab) produced in vivo were significantly lower in transgenic mice compared with the normal control mice at respective immunizing doses of KLH. In addition, a little or no anti-KLH Ab production was detected in culture supernatants of KLH-primed transgenic mice spleen cells. KLH-primed T cells from normal and transgenic mice induced anti-KLH Ab production from transgenic B cells in the presence of antigen-presenting spleen adherent cells (SAC) from normal mice, but not those from transgenic mice. Depletion of dendritic cells from normal mice-derived SAC completely abrogated the anti-KLH Ab response in transgenic spleen cell culture and their addition to the culture restored the response. Low efficiency of transgenic dendritic cells was demonstrated in sodium periodate (NaIO4)-induced non-specific and allogenic antigen-induced T-cell proliferation. Finally, cytofluorometric analyses showed a reduced Ia antigen expression on transgenic dendritic cells. These results indicate that low responsiveness of transgenic mice in specific-antibody response is not due to functional defects in T cells or B cells but rather to a defect of antigen-presenting activity of dendritic cells.