Sheep and human erythrocytes, partially processed by Staphylococcus aureus or Clostridium perfringens, were susceptible to lysis in the presence of Propionibacterium acnes. P. acnes liberated a lipase that was detected on Tween 80 agar and also on phospholipase C-precipitated egg yolk agar. Such a lipase might have contributed in the process of an intensified cellular lysis. Similar reactions were attempted with Lactobacillus acidophilus, known to possess a nondiffusible lipase, and failed to produce any such reactions. The synergistic reactions, between P. acnes and C. perfringens, were compared with The classical CAMP reaction in an attempt to find a correlation with the established membrane composition of the erythrocytes involved. Synergistic reactions observed do seem to reflect the membrane composition. Such findings, besides being contributory to an understanding of the role of these organisms in the process of pathogenesis, are of importance in the elucidation of molecular organization of biomembranes. Detailed studies, involving a large number of representative anaerobic bacteria, may also help provide an avenue in anaerobic species identification.