1. Clinical, hematological, genetic and peripheral blood globin synthesis studies were carried out on 17 symptomatic Brazilian thalassemics and their parents who live in the northeast of São Paulo State. The group included 8 beta zero-thalassemia homozygotes, 7 carriers of at least one beta + gene, one delta beta zero/beta zero-thalassemia double heterozygote and one beta zero homozygote also carrying the alpha-chain variant Hb Hasharon (alpha 2(47) His beta 2). 2. The mean non-alpha/alpha ratio for globin biosynthesis of the patients lacking HbA (beta zero homozygotes and delta beta zero/beta zero double heterozygotes) was 0.26 +/- 0.11 (mean +/- SD), which is not statistically different from the value of 0.32 +/- 0.06 obtained for the carriers of at least one beta + gene. In contrast, the mean non-alpha/alpha ratio for the thalassemia major patients (0.22 +/- 0.07) was significantly lower than that obtained for the milder cases (0.34 +/- 0.06) although the beta/alpha ratios for the parents of the two groups were similar. 3. The heterogeneity within this group of Brazilian patients having two thalassemic genes, i.e. 60% who are beta zero homozygotes and 40% who are carriers of at least one beta + gene, is consistent with the Italian origin of most of these patients.