Spleen cells were prepared from Balb/c mice immunized 30 days previously with alum-precipitated ovalbumin (OA), which manifested high, persistent titres of anti-OA IgE and IgG. The adoptive transfer of 5.0 X 10(7) such cells to X-irradiated syngeneic recipients produced comparable persistent IgE/IgG responses, in the absence of secondary antigenic challenge. Fractionation procedures indicated that the nylon-wool adherent population from the spleen was the most active in effecting transfer of the response. However, donor T-cells were also required, as pretreatment of the cellular inoculum with anti-Thy 1.2 antiserum ablated transfer. The inclusion of serum containing anti-OA IgG (but not IgE) in the cellular inoculum also blocked the transfer.