It has been reported that the stability of a 1.11-GBq (30 mCi) technetium-99m d,l-hexamethyl-propylene amine oxime (HMPAO) preparation can be improved to up to 5 h by the addition of 200 micrograms CoCl2.6H2O within 2 min after reconstitution. However, it is not clear whether this method is also efficient for high-activity preparations (5.55 GBq) and whether this modified 99mTc-d,l-HMPAO has the same biological properties and can safely be used. We have now studied CoCl2-stabilised 99mTc-d,l-HMPAO preparations containing different amounts of "in-house" HMPAO ligand and SnCl2 and reconstituted with activities from 1.11 GBq to 5.55 GBq 99mTc. The characteristics of the generator eluates were also divergent, ranging from fresh eluates from a generator eluted less than 2 h previously to 4-h-old eluates from a generator not eluted during the preceding 72 h. Preparations containing up to 5.55 GBq 99mTc and as low as 2 micrograms SnCl2.2H2O can be efficiently stabilised for at least 6 h by the addition of CoCl2 shortly after reconstitution. Interestingly, it was found that the stabilisation method is not efficient if the cobalt ions are added prior to reconstitution of the preparation. This implies that the cobalt chloride cannot be incorporated in the labelling kit. Also, preparations with amounts of the ligand lower or higher than 0.5 mg formed the 99mTc-d,l-HMPAO complex with low radiochemical yield or showed rapid degradation. Therefore, combination of a subdivision and storage of Ceretec kits in fractions (as reported in the literature) is contra-indicated with this CoCl2 stabilisation method. CoCl2-stabilised Ceretec kits reconstituted with 5550 MBq 99mTcO4- and used 4-5 h after preparation retain the diagnostic usefulness of the fresh 1110-MBq preparation with regard to leucocyte labelling and brain imaging. Although baboon brain uptake of the stabilised preparation was 6%-9% lower, this small difference could not be distinguished in the tomographic images. The data obtained with both inhouse prepared d,l-HMPAO and Ceretec kits suggest that the eluate restrictions recommended by the Ceretec manufacturer can be neglected if the preparation is stabilised with Co2+ ions. Studies with 57Co-spiked CoCl2 added to d,l-HMPAO preparations demonstrated that the Co2+ ions clearly interact with the d,l-HMPAO ligand, probably to form one or more complexes. From biodistribution studies in mice it became evident that the toxicological profile of the Co2+ ions in the presence of d,l-HMPAO should be more favourable than that of cobaltous ions. For these reasons, it seems justifiable that CoCl2-stabilised 99mTc-d,l-HMPAO preparations should undergo rigorous studies to elucidate their clinical usefulness and pharmacological safety.