UNLABELLED The objective of this work was to study the potential interference of propanolol, cyclosporine, adriamycin, and nifedipine with the in vitro labelling of erythrocytes. MATERIALS AND METHODS 20-ml blood specimens were collected from 40 healthy volunteers who had not taken any drug in the week before collection. 2.0-ml alliquots were incubated at 37 C with different concentrations of the drugs for 30 minutes. In all cases, a control group was incubated with saline (0.9%). Two labelling methods were used: 1) EDTA method, in which 60 l of SnCl2 (10.2 g/ml), 2.0 ml of saline, 0.2 ml EDTA 2.2%, and 7.4 MBq of Na99mTcO4 were added, and the mix was incubated for 5 minutes. 2) The hypochlorite method, in which specimens were initially incubated with SnCl2 (10.2 g/ml) for 5 minutes. Then, 40 l of 1% hypochlorite and all the reagents described in the previous method were added. With both methods, erythrocytes were separated by centrifugation and the labelling yield was estimated. RESULTS RESULTS did not show significant differences between the yield of the control group labelling and the yield of the different concentrations of the tested drugs. Also, significant differences were not observed the two labelling methods used. CONCLUSION Both labelling methods are useful for the in vitro preparation of 99mTc-erythrocytes. The absence of significant differences in the labelling yield indicates that in vitro interferences observed by some investigators are associated with concentrations exceeding therapeutic plasma levels. On the other hand, the reported in vivo interferences might be due to the presence of active catabolites and/or interference between different drugs.