A new serially transplantable human prostatic cancer (HONDA) in nude mice was established from a patient with metastatic prostate carcinoma. The tumor grows well in male nude mice. Doubling time of the tumor weight at passage #13 was 9.5 +/- 0.87 days (mean +/- SD). The tumor retains the original histological features of adenocarcinoma even after 6 years of continuous passage. High levels of human prostatic acid phosphatase were detected by radioimmunoassay in sera from the tumor-bearing mice. The tumor cells contain human prostate specific antigen. Electron microscopy showed particles resembling type A retroviruses in cisterns of endoplasmic reticulum, and particles resembling type C retroviruses in the intercellular space of the tumor cells. The tumor grew well in female mice treated with testosterone, but not in untreated female mice or castrated male mice.