Nude mice of NIH/Swiss background were utilized for the heterotransplantation of a tissue culture cell line derived from a human prostate adenocarcinoma metastatic to the brain. These cells, which had been grown in vitro for 13 passages, formed solid tumors when injected s.c. into nude mice. The cell line DU 145 has been passaged 60 times in vitro over a period of 18 months. Tumors removed from the mice were serially transplanted to additional mice and reestablished in vitro. Light-microscopic analysis of the tumor grown in nude mice revealed a strong similarity to the patient's metastatic tumor. The ultrastructure of the tumor cells propagated in nude mice was compared to that of the original human tumor cells and to the tissue culture cells, both before and after passage in nude mouse. No major differences were detected. Karyotypic analysis of the tumor cells grown in vitro before mouse passage, grown in nude mouse, and grown in vitro after mouse passage indicated chromosomal identity and consistent marker chromosomes: three large acrocentric chromosomes and metacentric minute chromosomes.