Abstract Near-ultraviolet survival of colony forming ability was compared between fibroblast strains from normal individuals and seven strains from Bloom's Syndrome patients using monochromatic light at 313 nm. Six BS strains possessed abnormal survival properties. Two strains lacked the low dose shoulder, which is characteristic for normal fibroblasts, but their overall sensitivity was in the normal range. Four BS strains were hypersensitive and possessed biphasic survival curves with a sharp initial drop. They mimick in culture the characteristic solar sensitivity of BS patients.