SUMMARY: A number of media have been developed for the production of micro- coccin. A satisfactory medium consists of corn steep liquor to 1800 mg. N/1. with an aqueous extract of dried autolysed yeast to 1800 mg. N/l., which gives titres of the order of 40-6Opg. micrococcin/ml. in 3 days. A soluble yeast extract such as Marmite may be used in place of the dried autolysed yeast. Moderate aeration is required for good titres but initial pH is relatively unimportant, any value in the range 6.0-7.0 being suitable. The main energy source in sugar-free media is probably the aerobic oxidative deamination of amino-acids. An inoculum that has been pre-incubated gives better results than an inoculum used immediately after it has been prepared. The addition of various inorganic ions to the medium was tried without any detectable influence on titre. The production of micrococcin (Su, 1948) in quantity was undertaken to provide material with which other workers could examine extraction of the active substance and its behaviour both in vitro and in vivo. The principal object of the present experiments was to reduce the cost of the medium and to supply information necessary for fairly large-scale fermentation. Although a number of ad hoc experiments were carried out in shake flasks, stirred aerated flasks were used for most of the work. The correlation between the results obtained in stirred aerated flasks and those obtained in a 100 gal. stirred aerated fermenter was good. METHODS Stirred aerated Jlasks. The stirred aerated flask (Fig. I), designed to study general fermentation problems, has been in use for more than 3 years. During this period a number of fungi have been examined. No diffic'ulties, apart from control of foaming, were found in getting a sufficient supply of oxygen to the fungi examined, some of which had an extremely high oxygen demand. The basis of the design is a bolt-head Chance Hysil flask of 5 1. capacity, normally containing 4 1. of medium. The flask is fitted with a black rubber bung having five moulded holes through which pass the aerators, stirrer, sampling arm and the air outlet filter. The rubber bung is retained in the flask by a suitably perforated metal plate clamped to a metal ring round the neck of the flask by butterfly nuts. The aerator arms, which have a bore of &in., are not restricted at the nozzles. They carry baffle plates preventing vortices and increasing the agitation effect. A relatively small stirrer facilitates assembly. It operates above the aerator outlets and disperses the air in the medium. A range of stirring speeds from 100 to 1000 r.p.m. is available. Greater agitation can be obtained by the addition of links of stainless steel chain to the blades of the stirrer, but excessive speeds with such chains have