Since the homogeneity factor (HF = first h.v.l./ second h.v.l.) might be useful in specifying diagnostic x-ray beam quality for radiation safety purposes, a study of HF data for pulsating potentials was undertaken. Aluminum, of 2S grade, transmission curves were determined for a half-wave therapy apparatus with a Thermax tube, and for a full-wave diagnostic machine with a Dynamax 20 tube. These tubes have inherent filtration equivalent to 0.5 mm Al. Visualization of beam wave-forms with a photo-multiplier showed little smoothing effect of cable capacitance. Potentials were determined by the K-level excitation method. The focus-chamber-distance and the irradiated area were chosen so that the measured h.v.l. values were within 1 per cent of the unique h.v.l. Appropriate corrections were made for the energy dependence of the measuring chamber. Table I shows that the steeper target angle of the diagnostic tube, as compared to the therapy tube, increases both the first and second h.v.l. at a given kVp. The h.v.l. values for both tubes change as expected with varying filter and kVp, but the rate of change is such that the HF (Table II) is essentially the same for both tube types, except at low potential and low filter; the greater proportion of lower energy x rays results in a steeper, straighter attenuation curve and a greater HF for the therapy tube. Agreement with constant-potential therapy-tube data taken from the literature (1, 2) (Table II) seems fortuitous; apparently the greater effective filtration of the steep target angle of a diagnostic tube tends to compensate for the higher average energy of the constant potential beam. Agreement with other therapy-tube data at higher potentials is reasonably good. The change in HF with potential for a given filter is very gradual when the potential is above 80 to 90 kVp. Conclusion: The HF is not a useful specification of diagnostic beam quality since it does not provide a means for distinguishing between x-ray beams of widely varying characteristics.