The effect of treatment with 8-methoxypsoralen (8-MOP) plus near-UV radiation (UVA) or with X-rays on the rate of DNA semi-conservative synthesis of fibroblasts from 10 Fanconi anemia (FA), two heterozygous, and three normal cell lines was studied. Following treatments with either X-rays or low doses of 8-MOP plus UVA leading to a majority of monoadducts over cross-links per genome, the FA and heterozygous cell lines were indistinguishable from normals: the transient inhibition of semi-conservative DNA synthesis was followed by the recovery of a normal rate of synthesis. In contrast treatment with higher (but not saturating) doses of 8-MOP plus UVA allowed us to distinguish two classes among the FA cell lines. One class demonstrated a pattern of recovery similar to that of heterozygous and normal cell lines. This indicates that in such cell lines, the predominant lesion in this condition, the cross-links, do not arrest DNA synthesis and are likely to be normally repaired. Another class of FA cell lines did not show a recovery of a normal rate of DNA synthesis even after prolonged post-treatment incubation and although the proportion of cells in S phase was similar to that of the strains of the first category. This indicates that in such cell lines the repair of cross-links is inhibited at some step which is not necessarily the incision one.