SummaryThe effectiveness of the classic methods for red blood cells (RBCs) labelling with 99m Tc has been demonstrated in nuclear medicine. However, nuclear physicians have found, in certain circumstances, this diagnostic technique fails and poor quality images are obtained. In this work we report on an alternative method that is of useful in these occasions, for in vitro labelling RBCs with 99m Tc-d,l-HMPAO complex. The study shows a high and reproducible labelling efficiency (94.14 AE 0.38), using low amount of tin. The RBCs were isolated from plasma and other interfering blood cells before adding 99m Tc-d, l-HMPAO. The tracer was retained and the elution rate from RBCs was low (less than 6% after 120 min). The preclinical results indicate that this new method could be a good alternative to the standard classic methods for specific cases.