Mesenteric lymph node cells from normal rats and rats infected with Nippostrongylus brasiliensis (Nb) were cultured with pokeweed mitogen (PWM) or Nb antigen, and the development of IgM-, IgG2a-, or IgE-containing cells was assessed by immunofluorescence. Normal lymph node cells stimulated with PWM developed into both IgM- and IgE-containing cells, whereas similar stimulation of cells from Nb-infected rats resulted in the development of IgM-, IgG2a-, and IgE-containing cells. The in vitro plasma cell response to PWM was dependent on the presence of T lymphocytes. Lymph node cells from Nb-infected rats responded to Nb antigen and developed into plasma cells of IgM, IgG, and IgE classes. The response was antigen specific and required antigen-primed T cells. Depletion of IgE-bearing cells of IgM-bearing cells before stimulation with either PWM or Nb antigen diminished the level of IgE-forming cell development, suggesting that IgE-IgM double bearing cells are precursors of IgE-forming cells. The distribution of the three isotypes among the Ig-forming cells that developed in response to PWM was influenced by the source of both B and T cells. When B cells from Nb-infected rats were employed as a source of precursors, T cells from infected animals were more effective than normal T cells for the development of IgE-forming cells, whereas the latter cells were more effective for the development of IgG2a-forming cells than T cells from infected animals.