Fragile sites on murine chromosomes were induced by the antimetabolites methotrexate (MTX), fluorodeoxyuridine (FdU), and aphidicolin (APD). To facilitate chromosome identification the analysis was performed on chromosomes of a CD/CD mouse that possesses nine pairs of Robertsonian translocation chromosomes of known arm composition. The pattern of induced fragile sites was rather similar for the different antimetabolites used. Many of them, e.g., 2B, 3B, 5B and 9D, are included in Giemsa-negative bands. On the X chromosome a fragile site was mapped to the region XC/D. Comparative mapping data with human fragile sites have been informative in most instances. Conservation of synteny within known linkage groups seems very likely.