Abstract: The present invention relates to methods of treating or preventing cancer by administration of human chorionic gonadotropin, .beta.-human chorionic gonadotropin or a peptide containing a sequence of a portion of .beta.-human chorionic gonadotropin. In a preferred embodiment, the invention provides methods of treating or preventing Kaposi's Sarcoma, breast cancer or prostate cancer. in another preferred embodiment, the invention relates to .beta.-human chorionic gonadotropin peptides for treatment or prevention of cancer. The invention further provides assays for the utility of particular human chorionic gonadotropin preparations in the treatment or prevention of cancer. Pharmaceutical compositions and methods of administration are also provided.