Following intensive comparative investigations of Priodax and Telepaque in varying doses under controlled circumstances for evaluating both the clinical side-effects and the roentgenographic efficacy, it was determined that, for routine use in a large institution, Telepaque in 2-gm. dosage constituted our cholecystographic medium of choice (1, 2). The subsequent introduction of Teridax necessitated further comparative studies to assess the possible advantages of this newer medium under identical conditions. Attention has been called to the potential disadvantages of Telepaque, namely, the presence of unabsorbed or excreted contrast material in the colon, which might obscure a portion of the gallbladder, and the intense opacification, which might obscure included stones. Inasmuch as Teridax is excreted almost completely in the urine and does not result in as dense gallbladder opacification, early reports emphasized the advantages of this medium in avoiding these difficulties. The chemical structure of Teridax—3 (3 hydroxy-2,4,6 triiodophenyl) 2-ethyl propionic acid—and Telepaque—3(3-amino-2,4,6 triiodophenyl) 2-ethyl propionic acid—indicate that they are closely related chemically and have approximately the same iodine content. The tabulation of side-effects was accomplished by questioning patients in conjunction with the taking of a history just prior to combined cholecystography and upper gastrointestinal examination. The resultant cholecystograms were evaluated according to the scale used in previous studies, the gallbladder shadows being classed as excellent, good, faint, or nonvisualization, with a notation as to the type of stones demonstrated. No side-effects were noted in 110 (71 per cent) of 155 cases. This is in fair agreement with the 77.5 per cent patients without side-effects in an early series elsewhere (3). The side-effects encountered in the present study are shown in Table I, where they are compared with those observed in the author's previous series with Telepaque (2). The side-effects of Teridax grouped under “other” include headaches in 3 cases, and dizziness, epigastric burning, gas, and abdominal cramp in 1 each. It is noted that the incidence of nausea and dysuria is slightly higher than in Teridax series reported by others (3, 4), but it is to be emphasized that these manifestations were mild in degree and that evaluation of such subjective sensations are difficult and may be considerably influenced by the manner of interrogation, which was the same as used in tabulating the comparative values for other media. To be noted is the slight overall advantage of Teridax in the number of asymptomatic patients, the slight increase in mild nausea with Teridax, and the more marked decrease in diarrhea with Teridax.