1. Seven strains of Strept. mucosus are described which have the following characteristics: large “mucous” colonies on serum agar and other media referred to in the text, with a subsequent particular form of lysis; no growth occurred on gelatin at 22° C., although the organisms were still viable on this medium after 10 days at this temperature; inulin was fermented and milk acidified without any clot formation; the organisms were only partially soluble in bile.

2. All our strains were isolated from patients suffering from acute infections of the respiratory tract.

3. Inflammatory and suppurative lesions were produced in rabbits and guinea-pigs by inoculation of the live organisms.

4. Vaccines may prove highly toxic when injected subcutaneously even in small doses into patients suffering from these infections.

5. Immune sera containing precipitins were formed in rabbits inoculated with the live organisms, but not when filtered toxins or dead cultures were employed.

6. Precipitin antigens were obtained by high-speed centrifugalisation of serum-broth cultures until the supernatant fluid was glass clear, and from centrifugalised suspensions in saline from egg cultures, but after filtration of the cultures through a Seitz filter or Berkefeld V candle the filtrates were inert.