The gene frequency for (sup -SEA/) deletional alpha -thalassaemia is high in Southeast Asian populations. We report a simple immunocytological test that is highly sensitive and specific for the detection of adult carriers of the (-) SEA/) deletion. We prospectively studied 206 consecutive adult blood samples. All 41 with the (-) SEA/) deletion had a positive test; all but 1 of the 165 non-carriers were negative. This test, whose major requirement is a fluorescence microscope, should be useful to identify couples at risk of conceiving fetuses with homozygous alpha-thalassaemia .