The presence in normal serum of inhibiting subsequences against Bacillus subtilis

Sera from 35 normal persons who had received no previous medication were tested for inhibiting substances against Bacillus subtilis and Staphylococcus aureus. Thirty of the sera (85 per cent) inhibited Bacillus subtilis in dilutions varying up to 1:32, but in no instance was Staphylococcus aureus inhibited. The data presented would indicate that Bacillus subtilis is not a suitable organism for use in the assay of antibiotics in the presence of serum.