SUCCESSFUL production of high-titre diphtheria toxin has already been described1, involving aluminium tanks containing 10 or 50 l. of medium. The inoculated medium was stirred at 1,400 r.p.m. so that a vortex was produced, and under these conditions high-titre toxin was obtained in 48 hr. The inocula for the tanks were produced in shake flasks2 which gave relatively poor results after 48 hr. shaking, and which could not in any way be compared with tanks. Thus no simple method existed which would give high-titre toxin in 48 hr. and which could be used on a scale small enough to allow the easy handling of large numbers of cultures in the laboratory. It was felt that some improvement might be expected if the aeration of the shake flasks was improved, and this led to the use of baffled shake flasks3. In the present work 1-l. Erlenmeyer flasks were used, into which single glass baffles (approximate dimensions, 50 mm. high, 25 mm. wide and 1 mm. thick) were fused. On a rotary shaker (140 r.p.m., ½ in. radius) these gave an oxygen uptake at the start of the shaking period of 120 ml. oxygen/100 ml./hr. as measured by the sulphite oxidation technique4. This compared with 30–40 ml. oxygen/100 ml./hr. for an unbaffled flask under the same conditions, and the difference was due entirely to the surface of the medium being continuously broken up on the baffle. When no baffle was present the surface of the medium as it swirled around in the flask remained unbroken and no air bubbles were incorporated into the liquid.