Mesenteric lymph node cells were obtained from rabbits primed with either dinitrophenyl derivatives of Ascaris antigen (DNP-Asc) or Ragweed Fr.D (DNP-Rag) included in aluminum hydroxide gel. The cell suspension was stimulated in vitro with a mixture of a free homologous carrier plus hapten coupled to an unrelated carrier, i.e., DNP-KLH, and anti-hapten antibody response was assessed. Under optimal conditions, the IgG antibody response to the mixture was 2- to 6-fold more than that obtained by DNP-KLH alone. The IgE antibody response was not obtained by stimulation with DNP-KLH alone; however, addition of free carrier to DNP-KLH resulted in a definite IgE antibody response. It was also found that DNP-Asc-primed lymphocytes treated with DNP-KLH were triggered by cell-free supernatant obtained from the culture of DNP-Rag-primed cells activated by free Rag. The results indicated that enhancing soluble factor was released from carrier-specific cells upon stimulation with carrier. The lymph node cells primed with DNP-primary carrier conjugate and a secondary carrier by supplemental immunization showed antihapten antibody response to a mixture of DNP-KLH plus free secondary carrier. In this system, it was found that the enhancing effect of soluble factor correlated with helper function of T cell2 population specific for secondary carrier. When the carrier-specific cells had helper function for both IgG and IgE antibody responses, stimulation of the cells with free carrier resulted in the release of soluble factors which enhanced both IgG and IgE antibody responses. When helper functions of the carrier-specific cells were limited to the IgG class, however, soluble factor released from the cells enhanced IgG but not IgE antibody response. The results suggested that helper cells for IgG antibody response and those for IgE antibody response release different soluble factors which trigger hapten-primed precursor cells for the respective immunoglobulin class. Evidence was obtained that enhancing soluble factor directly triggers hapten-primed precursor cells rather than carrier-specific helper cells.