THERE are few reports in the literature of immunological tolerance induced by non-immunogenic substances. In 1965 Maurer et al.1 studied the effects of neonatal injection of synthetic polymers on the subsequent immune response of adult rabbits against related cross-reacting polymers. They found that the injection of poly-L-glutamic acid caused a depression in antibody response when animals were immunized with a co-polymer of glutamic acid and alanine in spite of the fact that the poly-L-glutamic acid was not of itself capable of immunizing rabbits. Bauminger et al.2 subsequently showed that newborn rabbits treated with the non-immunogenic multichain poly-DL-alanyl-poly-L-lysine made no antibody specific for the poly-DL-alanyl determinants when subsequently immunized with poly-DL-alanyl protein conjugates. They concluded that “there is no correlation between the immunogenicity of a substance and its capacity to induce tolerance”. It should be noted, however, that in a re-test of the ability of the multichain polymer to elicit an immune response, they found a measurable antibody level in one rabbit and concluded that this material was a poor immunogen in rabbits.