Conventional and molecular cytogenetic analyses of three murine cancer cell lines that had been induced in male athymic mice Ly the injection of three different human prostate cancer cell lines revealed selective amplification of the Y chromosome. In particular, analysis of metaphase and interphase nuclei by fluorescence in situ hybridization (FISH) with the mouse Y chromosome-specific DNA painting probe revealed the presence of various numbers of Y chromosomes, ranging from one to eight, with a large majority of nuclei showing two copies (46.5-60.1%). In Interphase nuclei, the Y chromosomes showed distinct morphology, allowing identification irrespective of whether the preparations were treated for 15 min or for 5 h with Colcemid, a chemical known to cause chromosome condensation. However, FISH performed on human lymphocyte cultures with chromosome-specific DNA painting probes other than the Y chromosome did not reveal condensed chromosome morphology in inter-phase nuclei el en after 12 h of Colcemid treatment. Our FISH results indicate that (1) the Y chromosome is selectively amplified in all three cell lines; (2) the mouse Ti chromosome number is comparable in both interphase and metaphase cells; (3) the Y chromosome number varies between one and eight, with a large majority of cells showing two or three copies in most interphase nuclei; (;1) the condensation of the Y chromosome is not affected Ly the duration of Colcemid treatment bur by its inherent DNA constitution: and (5) the number of copies of the Y chromosome is increased and retained not only in human prostate tumor cell lines but also in murine tumors induced by those prostate tumor cell lines.