The effect of the castor bean toxin, ricin, on rat spleen and mesenteric lymph node cell cultures was tested. Rat lymphocytes were cultured for 2 days in the presence of optimal doses of the mitogens phyto-haemagglutinin, conconavalin A, and lipopolysaccharide. At concentrations of 10––3 to 10––5 mg/ml ricin, spontaneous and mitogen-driven proliferation were inhibited. At lower concentrations, spleen but not mesenteric lymph node cells showed enhanced proliferation. Studies are continuing to identify subpopulations of lymphocytes with increased or decreased sensitivity to ricin and their role in the regulation of IgE.