The 16-kDa prolactin (PRL), derived from the proteolytic cleavage of wild-type 23-kDa PRL, has been shown to have antiangiogenic activity. Such an antiangiogenic activity may have an effect on tumor growth in vivo. Here we examined the effect of 23-kDa and 16-kDa PRL on tumor growth, and the potential of using recombinant 16-kDa human PRL for prostate cancer therapy. The effects of 23-kDa PRL and 16-kDa PRL on the tumorigenicity of prostate cancer cells in vivo were studied. Using an adenovirus transfer vector to achieve high efficiency 23-kDa and 16-kDa PRL transfection in DU145 and PC-3 human prostate carcinoma cell lines, we demonstrated that expression of 16-kDa PRL in the prostate cancer cells markedly reduced their ability to form tumors in a xenograft animal model. These studies established that the 16-kDa PRL has antitumor activity in vivo, presumably as a result of its antiangiogenic effect. Interestingly, 23-kDa PRL showed a weak and transient suppression of prostate tumor growth. The weak antitumor activity of 23-kDa PRL may be because of the production of 16-kDa PRL from 23-kDa PRL by the tumor cells. Thus, the apparent effect of 23-kDa PRL on the growth of DU145 and PC-3 cells in vivo may result from the combined effects of 23-kDa PRL and 16-kDa PRL. These results suggest that the 16-kDa PRL has potential as a treatment agent in prostate cancer.