Hematological and hemoglobin composition data are presented for 14 members of a Surinam family (and for 1 unrelated subject) with either a β-thalassemia heterozygosity [5 with the -29 (A – G) β+ mutation and 5 with the IVS II-849 (A→G) β° mutation] or a compound heterozygosity (the 5 remaining patients). Identification of the mutation was by hybridization of amplified DNA with 32P-labelled synthetic oligonucleotides. The data indicate distinct differences between the two groups of heterozygotes, mainly in degree of microcytosis and hypochromia, in Hb A2 level, and in the level of Gγ (high in the -29 heterozygotes and low in the IVS II-849 heterozygotes). The 5 compound heterozygotes had a thalassemia intermedia with high Hb F levels (high Gγ), elevated Hb A2, and Hb A levels comparable to those seen in patients with a homozygosity for the -29 mutation or with the combination of this β+-thalassemia and Hb S. An α-thalassemia-2 heterozygosity (-3.7 kb deletion) was present in 2 patients. Their hematological data were improved over those for the patients with four α globin genes; one was the mother of two sets of twins. The high Gγ value in the Hb F of the compound heterozygotes suggests that the high Hb F production in the condition is mainly directed by the chromosome with the – 29 (A – G) mutation.