A variant, HCT-8R, of the colon adenocarcinoma cell line HCT-8 was located and cloned. The variant and parent cell line were characterized with respect to morphology, growth characteristics, karyotype, production of Cea, and ability to form tumors in nude mice. The variant cells differed from the parent cells in morphology, marker chromosomes, and ability to form colonies in soft agar and produced more carcinoembryonic antigen. The two cell strains were equally oncogenic in nude mice, although HCT-8R cells produced poorly differentiated tumors while HCT-8 cells produced tumors with both differentiated and poorly differentiated areas. Thus, in nude mice, no correlation was observed between carcinoembryonic antigen production by cells and their oncogenicity.