99mTc-labelled macroaggregated albumin particles prepared from a commercial "kit" have been tested in detail. With the introduction of the method of gel chromatography column scanning it became possible to make fast and simple quantitative measurements of the radiochemical purity. This was found to be the only reliable method for quantitative determination of non-aggregated 99mTc-labelled albumin. The measuring results of the kit in question have shown a labelling efficiency of about 97--99%. In addition to a few percent of 99m-Tc-pertechnetate less than 0.5% of disturbing radioactivity was found. The labelling was stable for at least 5--6 h. About 80% of the particles are in the size-range of 10--80 mum. A rough estimation of the number of particles in the solution resulted in 0.4-10(6) per mg of MAA. The dynamic studies of the lung uptake and the elimination of 99mTc-MAA in man resulted in a biological half-time in the lungs of about 1--2 h. The radiation absorbed dose to the lungs per mCi administered 99mTc-MAA was estimated to 0.1 rad. Less than 1% of the radioactivity was accumulated in liver, spleen or kidneys during the first 25 minutes after injection.