Plasma and red cell folate mean contents have been found to be significantly lower in 41 symptom-free beta-thalassaemia heterozygotes than in 21 controls. Such decreases must be considered as effects of an increased folate utilization caused by the enhanced total, both effective and ineffective, erythropoiesis. Since no close correlation has been found between packed cell volume and plasma or red blood cell folate levels, it seems that in 'healthy' beta-thalassaemia subjects, the degree of anaemia is not influenced by folate body reserves. However, the frequent finding of reduced red cell folate contents suggests that further folate imbalance might lead to clinically significant degrees of folate deficiency.