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More than 35 million people living in the developing world are currently infected with the human immunodeficiency virus (HFV). The success of recent initiatives to provide care to these millions of people will depend on appropriate laboratory monitoring. This is presently unavailable in resource-poor settings due to the complexity and high costs of current laboratory equipment and assays. This work demonstrated that flow cytometry (FCM) can be utilised to provide cost-effective CD4+ T-cell enumeration for monitoring HIV disease. The novel tests standardised in this work comprised primary CD4 gating using one-colour protocols on single-platform FCM, and a combination of primary CD45 and CD4 gating using two- colour protocols on both single- and double-platform FCM. This new generation of affordable assays yielded absolute and relative CD4+ T-cell counts that were in close agreement with those generated by the more complex and costly state-of-the-art methods. The performance of all tests remained high even when generic monoclonal antibodies were used for staining cells. This thesis also showed that short-term fixatives can be utilised to stabilise whole blood samples for up to 10 days and therefore, contribute to the establishment of regional quality assurance programmes and intercontinental transport of specimens. All these advances are ready for use in laboratories of the developing world where FCM is available. A wider availability of FCM will, however, require inexpensive equipment. This work documented that an affordable flow cytometer operating with a red diode laser accurately discriminated lymphocyte subsets in whole blood. Inexpensive flow cytometers also efficiently performed leucocyte differential counts and read suspension arrays for infections such as HIV, measles and mumps. The advent of user-friendly, inexpensive and multitasking flow cytometers will provide the ideal cost-effective solution for the diagnosis, monitoring and surveillance of infectious diseases in resource-poor settings. This technology will be instrumental in improving health systems devastated by the HIV epidemic.