Investigators have used various techniques and radionuclides such as 51Cr and 32P-diisofluorophosphate to label blood cells and to study cell survival. Early studies also used these radionuclides to label human leucocytes for cell survival by in-vitro counting. But external imaging could not be done with these agents. Starting with the use of the gamma-emitting radionuclide (111)In-oxine for in-vitro labelling of phagocytic leucocytes, external imaging became possible. This method was the basis of visualisation of cell distribution within the body. Because an abscess consists primarily of leucocytes, leucocytes labelled with (111)In localise within the abscess and are detectable by imaging. Nowadays other radiopharmaceuticals with other underlying uptake mechanisms are also used to detect inflammatory or infectious foci in patients. Nuclear medicine can be most useful in patients with fever of unknown origin, where a focus has to be defined, or in patients where a lesion is known by clinical symptoms or by a radiological imaging and the differentiation between infection and other pathologies has to be made.