The incubation time required to attain 90% labeling efficiency (90% radiochemical purity) for technetium Tc 99m labeling of iprofenin (PIPIDA, Diagnostic Isotopes, Inc.), was investigated. Thirty iprofenin vials were prepared according to the manufacturer's instructions. Aliquots were withdrawn from 12 vials at times 0.5, 1, 2, 3, 5, 7, 10, 12, 15, 20, 30, 45, and 60 minutes after mixing. Aliquots were withdrawn from 18 vials at 1, 2, 3, 5, 7, and 10 minutes. Free pertechnetate and reduced/hydrolyzed technetium were isolated from aliquots with instant thin-layer chromatography and quantified with a scintillation well counter. Based on the 30 vials, a mean (+/- S.E.M.) of 5.83 +/- 4.32 minutes was required to attain 90% labeling efficiency. Using tolerance limits calculations, the incubation time required to be assured that 95% of vials have achieved 90% labeling is 16.8 minutes. It is concluded that 99mTc iprofenin vials should be incubated 17 minutes to be assured of 90% labeling efficiency.