Primary rat tracheal epithelium （RTE） cells can be transformed in vitro by a-particles irradiation. Oneout of 3 isolated morphologically trans formed colonies gave rise to an immortal cell line and ultimately be-came neoplastic by successive passaging. Cytogenetic analysis was performed on the cell line in order to un-derstand how the specific chromosome alterations participate in the process of neoplastic transformation ofRTE cells. The 5th

18th

39th passage cells and the colony cells isolated from soft agar （SA） culturing ofthe 4oth passage cells were analyzed. The results showed that the 2Oth passage cells were nontumorigenic

but the 4Oth passage cells were tumorigenic. The increase of the length of the long arrn of chromosome 15（15q+ ） was frequent at the 5th passage （67. 8% ）

especially in SA cells in which it occurred in a clonalfashion （100% ） Therefore

15q+ might be the critical gene lesion after a-particles irradiation. Mono-somy of chromosome 8 that occurred frequently at passage 5 （54% ） and 40 （53% ） might be correlatedwith earlier transformation of RTE cells. In addition

three marker chromosomes （Ml

M2 and M3）showed a passage-dependent increase’ In particular M1 and M2 that were highly frequent at passage 39（90% ） suggested that they might play an important role in the process of cell transformation.