Haematological characteristics have been compared in 29 subjects with heterozygous beta0 thalassaemia and in 33 subjects with heterozygous beta+ thalassaemia, identified by the type of sickle cell-beta thalassaemia among close relatives, in a Jamaican Negro population. Total haemoglobin, MCV and MCH were significantly lower in the beta0 type but the level of Hb A2 was not significantly different. Individual values for MCV, MCH and Hb A2 in the beta+ type occasionally overlapped those in the normal population casting doubt on the adequacy of these criteria in identifying all cases of heterozygous beta+ thalassaemia. The haematological differences are those which would be expected on theoretical grounds. The inability to confidently differentiate the two types of heterozygous beta thalassaemia has implications for genetic counselling. The inability to distinguish heterozygous beta+ thalassaemia from normals on any single haematological index suggests that surveys depending on estimations of Hb A2 or on MCV alone may have underestimated the prevalence of the beta+ thalassaemia gene.