The amount of type III pneumococcal polysaccharide required to cause immunological paralysis in sublethally irradiated mice or in mice treated with 6-mercaptopurine was less than that required to produce paralysis in untreated animals. In irradiated mice the paralysis persisted for the duration of the experiment, but after cessation of the 6-mercaptopurine treatment unresponsive animals, without further antigenic stimulation, made antibodies. Small doses of polysaccharide were antigenic when given 24 hours after irradiation.