The possibility that injection of mice with an affinity-purified goat antibody to mouse IgD (GaMδ) that stimulates polyclonal IgG1 secretion might also stimulate differentiation of B cells into IgE-secreting cells was suggested by the observation that such treatment induces T cells from those mice to secrete a lymphokine, B cell stimulatory factor 1 (BSF-1), that can stimulate both IgG1 and IgE secretion in vitro. Studies described in this paper show that injection of BALB/c mice with 200 to 3200 μg of GaMδ greatly increased the quantity of splenic ε chain-encoding mRNA, the number of spleen cells with cytoplasmic IgE, and the concentration of serum IgE 7 days after injection. Serum IgE levels obtained in these mice were approximately 100 times baseline levels and were comparable with those found in mice infected with the nematode parasite Nippostrongylus brasiliensis, but were approximately 2000-fold less than the peak serum IgG1 levels induced by GaMδ injection. Both IgE and IgG1 secretion in GaMδ injected mice were T dependent (blocked by anti-L3T4 antibody). These observations are consistent with the hypothesis that BSF-1 may play a role in the in vivo stimulation of IgE secretion and provide an easy to apply model for the investigation of in vivo regulation of IgE responses.