When cultured lymphocytes of a patient with 45,XY,t(13;18) (p11.1-2;p11.2) were treated with mitomycin C (MMC), dissociations of t(13;18) chromosomes were increased as compared with controls. Furthermore, various other anomalies in volving t(13;18), 13 and 18 chromosomes were observed, which seemed to have resulted from breakage-fusion-bridge (BFB) cycles due to MMC.