The effect on the rate of sister chromatid exchanges (SCEs) in Bloom syndrome fibroblasts by cocultivation with Fanconi anemia and xeroderma pigmentosum fibroblasts and with Bloom syndrome heterozygotes was studied. Cells of Fanconi anemia and xeroderma origin reduced the rate of SCEs in Bloom cells by about 45%-50%, just as control cells do. In contrast, heterozygous Bloom cells reduced the rate of SCEs by only 16%-28%. In absolute figures, Fanconi cells reduced the mean rate of SCE in Bloom cells from 55.7 +/- 5.50- to 27.7 +/- 6.44, xeroderma cells to 30.5 +/- 5.73, and control cells to 28.3 +/- 5.35. Three different cell strains from Bloom syndrome heterozygotes reduced the rate to 40.1 +/- 8.81, 47.0 +/= 6.94, and 47.5 +/- 8.32. There was no effect on any of these cell strains by Bloom syndrome fibroblasts. We interpret the functional deficiency of heterozygous Bloom syndrome fibroblasts as a gene dosis effect. It probably represents a specific manifestation of the yet unknown primary defect, because it suggests the existence of a "corrective factor" that is inactive or absent in homozygous Bloom cells and reduced in heterozygotes. It may be identical with or closely related to the normal gene product at the Bloom locus.