1. Antipneumococcic serum contains in addition to the antibodies against the various bacterial constituents, a non-antibacterial therapeutic factor.
2. The non-antibacterial factor has been separated from the antibacterial antibodies by absorption with homologous, heat-killed, virulent pneumococci.
3. The non-antibacterial factor is not type-specific; it is probably species-specific; it is not neutralized by concentrated pneumococcus broth culture filtrate.
4. The therapeutic effect of the non-antibacterial factor was demonstrated on the experimental, dermal pneumococcus infection of rabbits; this effect is demonstrable when it is administered per seor in conjunction with a certain subeffective dose of serum.
5. Evidence is presented for the assumption that the ultimate outcome of pneumococcus infection depends to a considerable extent upon this non-antibacterial factor.