1. The preparation and characteristics of a tellurite medium, containing, in addition to a peptone and Lemco agar base, a tryptie hydrolysate of casein, sodium lactate, L-cystine, sheep serum, an extract of sheep red blood corpuscles and 0·01% potassium tellurite, is described for use in the identification and isolation of Corynebacterium diphtheriae.

2. The colonial appearances of the three types of C. diphtheriae on the medium are described, and the distinguishing features between these and the colonial appearances of diphtheroids and other organisms liable to occur on pathological material are indicated.

3. Recommendations are made concerning what has so far proved to be the most suitable manner of inoculation of pathological material on to the medium.

The author would like to express his thanks to Prof. F. C. Happold for suggesting the problem, to Dr R. E. Jones for encouragement during the course of the work, to Miss K. Gilliver, M.A., for assistance in the preparation of the manuscript and to Miss G. M. Jenkins for technical assistance.