DU4475 is a new human breast cell line derived from a cutaneous metastatic nodule from a patient with advanced breast cancer. It has been in continuous culture for more than 1 year and has survived 140 subcultivations. The cells grow in suspension, displaying clusters and free-floating aggregates with serpentine-like outgrowths. The cell line is hypotetraploid with modal chromosome numbers between 86 and 93 and possesses five to six distinctive marker chromosomes. The cells grow readily in athymic nude mice and produce tumors from which cells can be reintroduced to culture in vitro.