1. Blockade of the reticulo-endothelial system by means of one intravenous injection of India ink as well as splenectomy did not alter the course of either Pneumococcus Type I infection or tetanus intoxication in mice. 2. The protective action of Antipneumococcus Type I serum against the corresponding infection, as determined by the injection of in vitro prepared mixtures of serum and culture, was definitely lower in mice which had received one blocking injection of India ink shortly before the test. 3. Titration of tetanus toxin and antitoxin in blocked and splenectomized mice gave results identical with those obtained in normal mice, if in vitro prepared and incubated toxin-antitoxin mixtures were injected. The degree of protection, however, conferred by a preceding dose of antitoxin against subsequent intoxication, was markedly lower in blocked mice than in normal control animals, this difference becoming more pronounced with the increase of the time interval.