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A study was made on the activity of various bacterial cell walls and peptidoglycans to liberate serotonin from rabbit blood platelets. All of the test cell walls or peptidoglycans prepared from 27 strains of 21 bacterial species were shown to cause a marked release of serotonin, regardless of differences in types of peptidoglycan and non-peptidoglycan moieties and in some biological properties. The assay made with the water-soluble "digests" of Staphylococcus epidermidis cell wall peptidoglycans, which were prepared by use of appropriate enzymes, revealed that a polymer of peptidoglycan subunits (a disaccharide-stempeptide) was definitely active in the release of serotonin, but a structural unit monomer was inactive. Among a variety of synthetic muramylpeptides and their 6-0-acyl derivatives, only 6-0-(3-hydroxy-2-docosylhexacosanoyl)-N-acetylmuramyl-L-alanyl-D-isoglutaminyl-L-lysyl-D-alanine was found to hold a strong serotoninliberating activity. Raskova et al. (12) and Ryc and Rotta (14) reported that peptidoglycans of Streptococcus pyogenes, Streptococcus pneumoniae, and Staphylococcus aureus lysed blood platelets of rabbits and rats and liberated serotonin (5-hydroxytryptamine). In the present study, we have investigated whether or not this activity is common to bacterial cell walls or peptidoglycans in general, by use of a number of cell walls which differ in the types of peptidoglycans and a nonpeptidoglycan moiety and in some biological properties. A study was also made of serotoninreleasing activity by water-soluble enzymatic digests of Staphylococcus epidermidis peptidoglycans, synthetic muramylpeptides, and their 6-O-acyl derivatives to elucidate the chemical structure responsible for the serotonin-liberating activity of bacterial cell walls. MATERIALS AND METHODS Cell walls and peptidoglycans. Cell wall specimens employed in the present study are listed in Table 1. Among them, the cell walls of Corynebacterium betae, Corynebacterium insidiosum, and Ampullariella regularis were a gift from H. R.

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