Human chromosome 11 was introduced into adenovirus-transformed baby rat kidney (BRK) cells by microcell-mediated chromosome transfer. The resulting microcell hybrids (MCHs) showed a reduced ability to form tumors upon s.c. injection into athymic mice. Further analysis, with the use of defined deletion chromosomes of 11p, indicated that the presence of region 11p13-p12 is necessary for the suppression of tumorigenicity. In contrast, the presence of region 11p15-14.1 appeared to increase the rate of tumor growth. Expression studies on the human Wilms' tumor I (WTI) and the insulin-like growth factor II (IGF-II) genes, which lie in regions 11p13 and 11p15, respectively, suggested the involvement of both genes in determining the degree of suppression of tumorigenicity. Finally, stable expression of a murine WTI protein in the adenovirus-transformed cells resulted in almost complete suppression of tumorigenicity, establishing the WTI protein as a tumor suppressor in this cell system.