Everyone who has been occupied with the weekly production of diphtheria toxin in quantity has been struck with the great variations which may and usually do occur in the strength of the toxin from week to week. It is indeed extraordinary as Madsen remarks (1908, p. 83) that a bacillus should for a long time give good and fairly constant results when grown in one laboratory and yet fail to do so when grown in another laboratory in a medium prepared with the greatest care according to the formula used in the first laboratory. That it is notmerely a question of medium is shown by the following experiment.A batch of bouillon was made up and inoculated in the usual way. Then half the flasks were placed in one of the two hot rooms we have here, and the other half in the other hot room. After nine days' incubation the cultures were treated as usual. Both lots were tested on the same day and it was found that while the M.L.D. of the one half was 1/150 c.cm., that of the other was 1/50 c.cm. Many and varied have been the procedures suggested for growing the diphtheria bacillus so as to obtain a highly potent filtrate and most of them have been tried in this laboratory but none can be depended upon to give a constant result. When discussing this point Dean (1907, pp. 467–468) gives the M.L.D. of a large number of toxins prepared under his direction, but they are notgiven in order as they were prepared and so one cannot get an idea as to how one batch differed from the next. I have not come across any such series in the literature of this subject and so it seemed that it would be of interest to put on record a two years' series of results in theorder in which they were obtained—more especially as a study of thisseries gave more food for thought than was anticipated.