Prostate cancer (PCa) is the second most frequently diagnosed cancer in men worldwide. Dialyzed Leukocyte Extracts (DLEs) are heterogeneous mixtures of low-molecular-weight peptides that improve clinical responses in various diseases. Here, we analyzed the effects of Transferon^TM, a commercial DLE with characterized active pharmaceutical ingredient and proven batch-to-batch reproducibility, in preclinical models of PCa. We employed v-Src-transformed murine prostate epithelial (PEC-Src) cells, which recapitulate the transcriptional profiles in human PCa, can be grown in immunocompetent mice, and consistently form bone and brain metastases. In vitro, Transferon^TM did not induce cytotoxicity nor alterations in migration /invasion of PEC-Src cells. In vivo, Transferon^TM reduced metastatic dissemination after intracardiac injection of PEC-Src and inhibited tumor growth of subcutaneous isotransplants. The antineoplastic effect of Transferon^TM correlated with changes in tumor infiltration, increased serum concentrations of IL-12 and CXCL1, and reduced levels of VEGF. Our results suggest that the antineoplastic effect produced by Transferon^TM is due to its immunomodulatory activity and not by a direct effect on cancer cells, and indicate that Transferon^TM could be beneficial as adjuvant therapy in PCa patients.