Summary1. Virulent staphylococci injected in mucin do not produce a characteristic leucocytic reaction in the peritoneal cavity. 2. Staphylococci injected intraperitoneally in mucin remain viable and spread from the abdominal cavity; without mucin, they are rapidly destroyed. 3. Staphylococcus vaccine suspended in mucin does not produce leuco-cytosis during the first hours after injection. 4. C. diphtheriae and meningococcus injected intra-abdominally are influenced in the same manner by the presence of mucin. 5. The leucocytic reaction of aleuronat is inhibited by simultaneous injection with mucin. A later injection of mucin does not destroy leucocytes already present in the peritoneal fluid. 6. The virulence-enhancing effect of mucin can be neutralized by leucocytosis previously induced by aleuronat. 7. All these observations suggest that mucin decreases the resistance of the host to bacterial invasion and imparts to microorganisms of otherwise low virulence the potentialities of highly invasive microbes.