We previously reported that the introduction of a normal human chromosome 1 via microcell-mediated chromosome transfer suppressed the transformed phenotypes, including anchorage-independent growth, of Kirsten murine sarcoma virus-transformed NIH3T3 (DT) cells. Soft-agar clones derived from DT-#1 cells (DT cells with an intact transferred human chromosome 1) exclusively failed to retain an intact form of this chromosome. Thus, a gene(s) with a suppressive activity on this chromosome had probably been lost. We therefore attempted to identify a commonly deleted region on human chromosome 1 in these soft-agar clones. Although eight of the 9 soft-agar clones examined still contained regions on this chromosome, to a greater or lesser degree, four loci on 1q21 and 1q23-q24 were commonly lost in all of them. Furthermore, the soft-agar clones had growth properties similar to those of DT cells. Thus, chromosome and DNA analyses suggested that human 1q21 and/or 1q23-q24 carries a transformation suppressor gene(s) which controls the transformed phenotypes of DT cells.