It is evident that the traditional laboratory examination for the diphtheria bacillus, based on the microscopic appearance of a stained smear from a culture on Loeffler's medium, is in every respect as subjective a matter as is the clinical diagnosis of the case by the physician. In each instance the decision is based largely on a visual image interpreted in the light of past experience with such images. It is true that the clinician has additional supporting evidence, involving touch, hearing, and even smell to assist him. The bacteriologist depends on sight alone, and his report will be correct in proportion to his earlier experiences in similar interpretations. Diptheria, however, has become such a rare condition in some areas that a generation of bacteriologists whose encounters with the organism have been relatively infrequent are replacing those of twenty years ago whose mornings were usually devoted to that particular matter.Moreover, it usually happens that the sudden appearance of a few cases of diphtheria in an area from which the disease has been absent for some time will result in a flood of throat cultures from contacts and possible carriers, as well as from all types of nondiphtheritic pharyngitis. The majority of these will be negative, but in each case a time-consuming search of a smear must be made before it can be so pronounced.These facts have resulted in the gradual substitution, in many laboratories, of a procedure which is much more rapid and somewhat simpler in subjective interpretation than the use of Loffler's cultures. This method depends on the inhibition by potassium tellurite of most throat organisms and on the rather characteristic colonial appearance of members of the Corynebacterium group, which grow well in its presence. An imposing array of formulae for tellurite media has already grown up in the literature of the past fifteen years. The writers hesitate somewhat to add one more to the list but are persuaded to do so because of the conviction that to be reasonably practicable a diagnostic medium4 should be relatively easy to prepare and reproducible in its results. The two portions of the medium to be described are commercially available, one in dehydrated form, the other as a fluid concentrate,2 and its preparation from this source is extremely simple. In the absence of a commercial source, the medium may still be pre-