1. Comparison of dose-response curves indicated that preimmunized animals were slightly more susceptible to the induction of immunological paralysis with pneumococcal polysaccharide than were normal mice. The results also indicated that the paralysis threshold was unaltered by preimmunization.
2. Transient desensitization of immunized mice could be achieved by an amount of polysaccharide far less than that required to induce paralysis.
3. A transient phase of weak immunity was detected prior to the onset of paralysis when induced by relatively low paralyzing doses of polysaccharide.
4. No "low dose" zone of paralysis (analogous to that obtainable with certain protein antigens) could be elicited with pneumococcal polysaccharide.
5. Massive proliferation of lymphoreticular tissues induced by Corynebacterium parvum failed to raise the threshold for paralysis induction, but amplified the immune response over the entire dose-response curve. Similarly, C. parvum failed to abrogate an established state of paralysis. The results suggest that the induction of polysaccharide paralysis is related to the concentration of antigen in the animal and is not modified by the number of immunologically competent cells.