The radiolabelling of blood cells is occasionally problematical. Difficulties occur either with the labelling process itself or after the labelled cells have been reinjected. Failure to label may be due to pharmaceutical factors, such as difficulties with collecting sufficient cells, sedimentation problems or instability of the cell chelator, or problems may arise which are patient-related, such as patient medication or the presence of disease. A number of surveys have been undertaken to assess the possibility of drug interference as a cause of problems with labelling or biodistribution. Leukocyte labelling difficulties occurred in patients who were on multi-drug therapy whose drug regimes included combinations of prednisolone, azathioprine, cyclosporin, ranitidine, nifedipine, cyclophosphamide and naproxen. While a direct causal relationship has not been established, the known adverse effects of the drugs on white cell function and kinetics suggest that patient medication could be an important factor in leukocyte labelling. Red cell labelling difficulties have occurred from time to time and published reports implicate pharmaceutical factors, such as choice of anti-coagulant, level of stannous ion and oxidation of technetium-99m (99Tcm). Patient-related factors such as the presence of disease or drugs have also been implicated. A survey of red cell problems and patient medication showed that difficulties occurred in patients who were treated with combinations of nifedipine, etoposide, idarubicin and cefataxime. In-vitro testing of a number of drugs with 99Tcm-labelled red cells has demonstrated that cyclosporin, nifedipine, verapamil, hydralazine, propranolol, digoxin and Teflon cannulae can affect labelling.