Chromosomal breakage was studied for its correlation to sister chromatid exchange (SCE) in cultured peripheral blood lymphocytes from normal individuals following treatment with different fragile site- and breakage-inducing chemicals. The average SCE per cell was similar for cultures untreated or treated with fluorodeoxyuridine (FdU), caffeine, and aphidicolin (APC) (6.5 SCE), but elevated with mitomycin C (MMC) (46.5 SCE). About half of the breaks induced with caffeine and FdU were at fragile sites, with fewer than 10% accompanied by SCE. Forty percent of MMC-induced breaks were at fragile sites, of which 47% had SCE. APC induced 68% breaks at fragile sites, of which 44% were associated with SCE. Fragile sites induced at > 4% included 3p14, 6q26, 7q22, and 16q23. the 3p14 fragile site was induced by all chemicals except caffeine. The different frequency in induction of fragile sites and their association with SCE by the chemicals tested suggest that they are related to different mechanisms of induction which may be dependent upon nucleotide content.