Rats immunized with a purified Ascaris suum allergen (Asc-1) or with its dinitrophenylated derivate (DNP-Asc-1) produced high levels of reaginic (IgE) antibodies. A second injection of antigen given 30 days later did not result in an anamnestic IgE antibody response. Immunization of adult-thymectomized, lethally-irradiated and bone-marrow reconstituted (ATxB) rats with soluble Asc-1 or DNP-Asc-1 failed to stimulate reaginic antibody production. The administration of glutaraldehyde-polymerized antigen induced in some but not all ATxB rats, low but detectable levels of IgE antibodies. These levels increased following a second injection of nonpolymerized antigen in A1 (OH)3 gels. Priming of animals with polymerized carrier and Bordetella pertussis did not stimulate a primary anticarrier IgE response but led to an enhanced antihapten IgE response following the administration of soluble DNP-Asc-1 in A1(OH)3. The results are consistent with the notion that a sharply reduced but clearly functional T-derived helper cell population could be triggered by the polymerized but not by the soluble form of the immunogen.