The rapid and reliable diagnosis of bacterial infection is crucial for three reasons: 1) delays in the identification of bacteraemia during the first 6 hours after hospital admission are associated with higher mortality rates; 2) delays in pathogen identification are associated with an increased utilization of hospital resources (e.g. for ICU treatment); and 3) the treatment of viral illnesses and non-infective causes of inflammation with antibiotics (because of inaccurate diagnoses) contributes to the development of antibiotic resistance, toxicity, and allergic reactions, all leading to increased medical costs. Therefore, the development of rapid and reliable diagnostic tests is an essential prerequisite for more accurate diagnosis and the effective use of antibiotics within hospitals and general practitioners’ surgeries. One of the most promising technologies likely to impact on rapid and reliable diagnostic testing involves the flow cytometric quantitative analysis of new specific and sensitive cell surface markers (receptors) of bacterial infection on phagocytes. As an example of the usefulness of this method, this chapter presents an outline of how flow cytometric quantitative receptor analysis can be utilised for the clinical differential diagnosis of hospitalized febrile patients suspected of having an illness of microbiological origin.