1. Sulfapyridine, administered to rabbits during the period of developing immunity after a single intravenous injection of heat-killed Pneumococcus Type I, exerted no influence upon the immune response. 2. Active immunity as indicated by increased resistance to homologous intradermal infection was present 48 hours after the immunizing injection and 2 days before circulating type specific antibodies were detectable. 3. Of the serological techniques employed for the detection of circulating antibody the mouse protective test yielded the highest percentage of positive results followed in order by tests for type specific agglutinins and precipitins, the latter being least satisfactory for the detection of small amounts of antibody.