Helper T cell-derived cytokines play a pivotal role in the production of antigen-specific IgG antibody by B cells. In order to examine the in vivo effect of massive activation of helper T cells on the production of specific antibodies, ovalbumin (OVA)-specific TCR transgenic mice (OVA23-3) were immunized with OVA and serum levels of antigen-specific antibodies were measured. As a result, a great enhancement of antigen-specific IgM secretion and delay of IgG secretion were observed while the T cells produced sufficient Th2 cytokines. Immediately after OVA-immunization, marked increase of serum IL-6 was noted, which was followed by a transient increase of antibody forming cells. Both in vivo and in vitro experiments demonstrated that excess amount of IL-6 enhanced IgM production by activated B cells while suppressing IgG production. These results suggest that overproduction of IL-6 in the early stages of the primary immunization promotes development of IgM producing cells and causes the delay of IgG1 secretion.