The suitability of two radionuclides (99mTc, 111In) for labelling lymphocytes have been evaluated in rats by comparison with a standard method using 51Cr. For the study of lymphocyte migration in patients labelling with 111In-labelled oxine is clearly the most promising because both in vivo and in vitro it remains associated with lymphocytes and the labelled cells migrate normally into lymphoid tissues. The physical characteristics of 111In are also favourable. Not only does 99mTc rapidly dissociate from lymphocytes but also it compromises their ability to recirculate from blood to lymph.