Three major radiopharmaceutical firms supply separated short-lived 99mTc as sterile sodium pertechnetate, ready for use in brain scans. For small laboratories whose work loads do not justify buying a generator, this “instant” technetium is very convenient. Within the territory covered by the various firms' delivery arrangements, material ordered before 2 P.M. is usually delivered before 8 A.M. the next day. In the spring of 1968 one of these firms announced that it would have to make a 20 per cent shift in its 99mTc calibration in order to be consistent with the calibrations of other suppliers and major laboratories across the country. In a small series of spot-checks on two suppliers (including this one), no indication of such a large discrepancy had been observed here. During the summer and fall of 1968, a series of tests was carried out on routine shipments of 99mTc from all three firms. These tests involved careful calibration comparisons using conventional dilution technics, as well as determinations...