Background: The secretory epithelial cells of the prostate gland use sophisticated vehicles named prostasomes to relay important information to sperm cells in semen. This prostasome-forming and secretory ability of the epithelial cells is also preserved in poorly differentiated prostate cancer cells. Aim: The aim of this thesis was to examine different characteristics of prostasomes, especially those derived from malignant prostate cells, linked to their potential role in diagnosis and prognostication of prostate cancer. Results: Serum samples of prostate cancer patients contained autoantibodies against seminal prostasomes in a higher concentration than did control sera. These autoantibodies were most frequently directed against 25 prostasome-associated proteins, but no one was prostate specific. Clusterin was one of the most frequently occurring prostasomal proteins. Elevated titers were however seen in both patients´ and control sera. Clusterin turned out to be a major antigen of seminal prostasomes. No prostate specific or prostate cancer specific protein was discovered upon proteomic analysis of prostasomes deriving from malignant cells of vertebral metastases of prostate cancer patients. Human chromosomal DNA was identified in both seminal prostasomes and PC-3 cell prostasomes and strong evidence existed that the DNA was localized inside the prostasomes. Four out of 13 DNA clones of seminal prostasomes featured gene sequences (31%). The corresponding figures for PC-3 cell prostasomes were 4 out of 16 clones (25%). Conclusions: Prostasomes are immunogenic and give rise to serum autoantibodies. The most frequently occurring autoantibodies were directed against 25 prostasomal proteins but none of these was exclusively prostate specific. Thirty different proteins were identified in prostate cancer metastasis-derived prostasomes but none of these proteins was prostate cancer specific. Human chromosomal DNA was identified in prostasomes of both normal and malignant cell origin.