In November 1961, the opportunity arose of developing a program of co-operative radiotherapy trials which had originated with the National Cancer Institute in the late 1950's. Fully documented, randomized radiotherapy with classification of disease was to be studied statistically. The first treatments of preoperative lung irradiation began in 1963. As part of the overall program it was thought essential to include a comparison of dosage at each center involved. A survey of the first group of therapy centers was undertaken; this paper presents the factors that were considered and the results obtained. A Victoreen r-Meter was carried to nine different centers to compare the method of calibration at those institutions, and to calculate the tumor dose in a standard phantom, comparing the local dosimeter readings and calculations with the transported Victoreen r-Meter. The results are presented in this paper. Correction Factors Timers and integrators should be discussed before considering the factors for the Victoreen. Timers used with x-ray and teletherapy machines may have errors of seconds; there may be errors due to the time required for voltage build-up. This factor has to be taken into account when exposures are short, as with a 25-r chamber. The difference between time set and time registered is usually constant, if the timer is set in the same manner each time it is used, and can be determined by exposing a chamber for different times and extrapolating to zero dose. Integrator systems also may have “end” effects which can be determined in a similar manner. The Victoreen r-Meter model 570 serial 359 employed in this study is one of four used at the M. D. Anderson Hospital. The following routine checks are made on their constancy. The electrometers are given voltage checks by measuring the voltage required to produce full and half-scale deflections; the chambers are intercompared monthly with x-rays of h.v.l. 1.26 mm. Cu. Intercomparisons are also made with radium, cobalt, and cesium gamma and x-rays or other half-value layers at less frequent intervals. Routine leak tests are performed on all chambers and desiccators are changed frequently. A model 70-5, 25-r chamber is normally used for cobalt-60 with Lucite build-up of 4 mm. wall thickness. At times of intercomparison 100-r and 250-r chambers which were employed in this work are also re-checked with suitable build-up caps. The correction factors are shown in Table I. Two of the four Victoreens at M. D. Anderson Hospital are nonlinear to the extent that it is worthwhile to make a linearity correction when the readings are beyond certain limits. For instance, a correction of +3.5 per cent is required for the instrument in this study, and −4 per cent for another for a full-scale deflection reading (Fig. 1).