Colony formation by erythroid burst-forming units (BFU-E) and erythroid colony-forming units (CFU-E) and the effect of hemin on colony growth was studied in vitro in three Finnish families with hereditary sideroblastic anemia (HSA). Defective activity of heme synthase has been demonstrated in family A and that of delta-aminolevulinic acid synthase in family B. No biochemical defect has been recognized so far in family C. CFU-E colony growth was defective in seven of the eight persons studied. The formation of BFU-E colonies was normal in family A and increased in family C, whereas of the two members of family B one showed normal and one decreased BFU-E colony growth. Hemin in 30-120 microM concentration increased significantly both BFU-E (p less than 0.01) and CFU-E (p less than 0.005) colony formation in family C. No effect was seen in family A, and in family B the only effect was normalization of the decreased BFU-E colony growth by the highest hemin concentration in one person. This study indicates that differences exist between families with HSA in erythroid colony formation and in response to hemin in vitro, but the low number of investigated members in each family does not permit a conclusive evaluation of the impact of the carrier versus patient status or of sex on the results.