Maturation of basophils/mast cells as well as formation of IgE antibodies are parts of the immune response to antigens. We have analyzed these important constituents of the allergic inflammatory response in vitro, cultivating human peripheral blood cells from allergic and normal individuals. Mesenteric lymph nodes were also cultured from two normals. In liquid cultures of peripheral blood cells, the number of small basophils initially present decreased and large basophilic cells appeared over a 2-week period. In normals, the maturation of large basophils from precursor cells could be stimulated with supernatants from peripheral blood cell cultures from atopic individuals stimulated with the relevant allergen. The IgE levels declined in cultures of peripheral blood, whereas the IgE antibodies in cultures of cells from a mesenteric lymph node from one of the nonallergic individuals increased during the 2-week period. In separate cultures, the IgE formation could not be reproducibly stimulated by antigen or mitogen. The study shows the possibility to stimulate the maturation of basophil/mast cell precursor cells present in the blood of allergic and healthy individuals with antigen-induced factors. In contrast, the IgE formation by blood cells was difficult to stimulate.