When human peripheral blood B cells are cultured for 6 days with the T cell-dependent peptide antigen ovalbumin (OA) in the presence of antigen-presenting cells and helper T cells, plaque-forming cells (PFC) are generated. These OA-induced PFC differ from the conventional high-rate antibody-secreting PFC formed after stimulation of B cells with recall antigens (e.g. tetanus toxoid) in that they secrete antibody at a very low level. Previous studies have shown that OA-induced PFC are B lymphocytes in an early activation state rather than cells that have differentiated into plasmablasts. The apparent arrest in the maturation of OA-induced PFC in an early activation phase can be overcome by simultaneous stimulation with interleukin 2 (IL-2) and gamma interferon (IFN-gamma). The isotype of the OA-specific antibodies secreted, however, are only of the IgM class, demonstrating that an isotype switch does not occur.