Rats were injected with 99mTc-Sn-diphosphonate preparation containing various concentrations of Al+3, ranging from 0 microgram/ml to 60 microgram/ml, and whole-body rat images were obtained at two hours postinjection to assess soft-tissue uptake. Uptake (kidney and liver) occurred at approximately 20 microgram/ml Al+3. [This is the maximum allowed in pertechnetate eluate by the Nuclear Regulatory Commission (NRC). ] Increased soft-tissue uptake and progressive degradation of bone scan images occurred with higher Al+3 concentrations. Chromatographic analysis confirmed the breakdown of the radiopharmaceutical preparation containing 20 microgram/ml or more Al+3. Millipore filtration showed no evidence of large colloid particles (greater than 0.2 micrometer).