The physical characteristics of 99mTc, including a short half-life of 6 h and a nearly monochromatic gamma emission of 140 keV, make it an excellent agent for scintillation scanning. Because of the short half-life and absence of significant beta emission, large amounts of activity may be used, the radiation dose to the patient being very low. Possibilities thus exist for greater resolution through optimal design of collimation and for more rapid scans because of better counting statistics. A colloid of this isotope has been prepared by passing hydrogen sulfide through a solution of 1 N HCl containing pertechnetate. This colloid, with an average blood disappearance half-time of 2.5 min, has proved useful in performing scintillation scans of liver, spleen, and bone marrow. A specially designed collimator of 721 holes has provided resolution comparable to the commercially available 31-hole collimator but with a sensitivity considerably greater than the 19-hole collimator. Maximum count rates over the liver are 75 000 to 100 000 counts/min following administration of 10 mCi of the colloid intravenously. Modification of a commercially available scanner has been made by bypassing the contrast enhancement circuits and doubling the speed in order to exploit the high count rates. Count rates over bone marrow are maximally 1/10 to 1/15 of the liver count, and spleen count rates fall in between liver and bone-marrow rates. Estimation of spleen size and extent of functioning marrow are possible in addition to detection of space-occupying disease of the liver.