Sister chromatid exchanges in chromosomes from human lymphocytes grown two replication cycles in medium containing 5-bromodeoxyuridine can be detected by fluorescence microscopy after staining with the bisbenzimidazole dye 33258 Hoechst. These exchanges are much more frequent than chromosome or chromatid breaks and appear to be partly but not entirely due to 5-bromodeoxyuridine incorporation. Sister chromatid exchanges are extremely sensitive indicators of chromosome damage produced by DNA cross-linking agents such as mitomycin C. Significant increases in the sister chromatid exchange frequency occur with 3 ng/ml of mitomycin C; higher concentrations of mitomycin C induce further sister chromatid exchanges. Comparatively few gross chromosomal aberrations are seen in cells exhibiting as many as one hundred or more sister chromatid exchanges. Most of the damage caused by mitomycin C to chromosomal DNA is apparently repaired without detectable changes in chromosome morphology. Analysis of sister chromatid exchanges may permit more sensitive detection of damage to DNA caused by other agents than has previously been possible by classical cytological techniques.