Several species of the genus Pleurotus have been found in this laboratory to form substances inhibitory for Staphylococcus aureus. Among these were two species, Pleurotus mutilus (Fr.) Sacc. and P. Passeckerianus Pilat, obtained from the Centraalbureau voor Schimmelcultures at Baarn. An antibacterial substance formed by these fungi was isolated in crystalline form from culture liquids; it was named pleuromutilin. P. mutilus grown on corn-steep, thiamine-peptone, or potato-dextrose agars for two days and tested by the streak-method, markedly inhibited Staphylococcus aureus, inhibited incompletely Mycobacterium smegma, and had no effect on Escherichia coli. Agar disks cut from colonies 10 days old formed inhibition zones 20 mm. in diameter with S. aureus and a small zone of incomplete inhibition with M. smegma. P. Passeckerianus produced similar zones of inhibition. Still Cultures.-P. mutilus was grown at 25°C. in 2800 ml. Fernbach flasks containing beech-wood shavings and a corn-steep medium.' About four weeks after inoculation the mycelium covered the surface of the liquid and the activity against S. aureus was about 512 dilution units per ml. Reflooding the mats with fresh corn steep medium resulted in as high activity in about one week after reflooding. The reflooding was repeated at about 10-day intervals until the mat became so thick that the operation was difficult. The flasks inoculated with P. Passeckerianus reached an activity of 256 dilution units per ml. in about one month.

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In a survey' conducted in this laboratory it was noted that 213 of the 332 species of Basidiomycetes studied produced substances that inhibited the growth of Staphylococcus aureus. Among the species that seemed worthy of further investigation was Pleurotus griseus.2 P. griseus grew with medium rapidity on a thiamine-peptone agar' and a modified Czapek-Dox agar to which corn steep solids had been added. It formed a pink pigment which diffused intio the agar. After the fungus had grown for one to several weeks, discs 5.5 mm. in diameter were cut on a radius extending from the center of the colony into the agar adjacent to the colony. These discs were placed on a yeast-peptone agar seeded with Staph. aureus and the plates incubated overnight at 37 4 2°C. Clear zones of inhibition were found around the discs indicating that some substance or substances had diffused from the agar discs into the surrounding seeded medium in sufficient concentration to inhibit the growth of the staphylococci in the agar. A disc cut from the agar at as great a distance as 20 mm. from the edge of the colony frequently produced a zone of inhibition. The zones, while small, were clear, indicating that bacteria resistant to the antibiotic substance or substances were absent. Disc tests, with strains of Staph. aureus resistant to other antibiQtic substances, indicated that P. griseus produced an antibiotic substance which was different from penicillin and different from the active substances produced by several other Basidiomycetest The antibiotic substance or substances from P. griseus did not inhibit the growth of Escherichia coli. P. griseus was grown on several types of nutrient media, differing from *one another in the source of nitrogen and carbon, to determine the effect of the medium on the production of the antibacterial substances by the fungus. Cane sugar, dark brown sugar, galactose, lactose, mannitol and corn steep solids were ineffective carbon sources; maltose and soluble starch were -poor; dextrose and levulose were the best and nearly equal so far as the 171