Haemoglobin A2 (HbA2) levels were determined on 25 beta-thalassaemia carriers by the microcolumn method and were found to range from 4.5-7.2 per cent (mean 5.2 +/- 0.82 S.D.). The haemoglobin level (Hb), mean corpuscular volume (MCV), plasma ferritin and HbA2 levels were measured on a further 299 consecutive Chinese pregnant women at a gestation of less than 24 weeks. 18 patients (6 per cent) had HbA2 level greater than 4.5 per cent and were diagnosed to be beta-thalassaemic carriers. It was observed that all these patients had a MCV below 75 fl. If this level is selected in a screening procedure based on measurement of MCV alone all beta-thalassaemia carriers could be detected and 11 per cent of the population screened would require HbA2 estimation. At a lower cut-off level of 70 fl, 8 per cent of the population screened would require HbA2 measurement (a decrease of 27 per cent) but the detection rate will be lowered considerably (83 per cent). The high false positive rate at all cut-off levels of MCV was largely due to the prevalence of iron deficiency anaemia in the population. Estimation of plasma ferritin level in patients with low MCV will reduce this false positive rate, but there will be a considerable delay in diagnosis in patients with concomitant iron deficiency and beta-thalassaemia. The presence of iron deficiency in beta-thalassaemia carriers did not reduce their HbA2 level below the diagnostic range in this study.(ABSTRACT TRUNCATED AT 250 WORDS)