The test for antineutrophil cytoplasmic antibodies (ANCA) was introduced into the author's laboratory in 1987. An improved indirect immunofluorescent method was developed, using a system which allows 16 instead of one serum sample to be screened on each microscope slide. The known disease associations of ANCA that have been explored include systemic vasculitis, renal limited vasculitis, chronic inflammatory bowel disease and HIV disease. In general the findings are similar to those which are emerging from other centres and confirm the value not only of the positivity but also the relevance of the intracellular disposition of the neutrophil cytoplasmic fluorescence in diagnosis. In this study 85% of patients with Wegener's granulomatosis were found to have C-ANCA. C, P and X-ANCA staining patterns were found in 57% of patients with ulcerative colitis. Forty one per cent of patients with symptomatic HIV have ANCA. Certain histological features such as neutrophil and vascular damage in invasive amoebiasis, and the established lytic effect of amoebae on neutrophils prompted the investigation of the possibility that ANCA may be generated in this disease. Seventy eight amoebiasis sera were screened and 98,70/0 gave a positive ANCA test with a pattern of fluorescence resembling that found in Wegener's granulomatosis. An ELISA test for specificity confirmed that, as in Wegener's granulomatosis, this amoebiasis-associated ANCA had proteinase 3 specificity. Of practical clinical importance is the fact that both HIV and amoebiasis are associated with a high level of ANCA positivity. These findings will need to be considered when ANCA tests are used in clinical decision making in an area where HIV disease and amoebiasis are endemic. A large number of normal volunteer blood donors have been tested and the false positivity rate of 0,5% confirms the specificity of the test. / Thesis (M.Med.)-University of Natal, Durban, 1993.