Summary Prostasomes are prostate-derived organelles in seminal plasma exhibiting pluripotent properties to facilitate the fertilization process. Seminal prostasome concentration, size distribution and expression of the prostasomal surface antigens CD10, CD13, CD26 and CD59 were examined by flow cytometry. The study group consisted of 79 men with involuntary infertility. Very strong correlations existed between the prostasome expressions of the different CD markers. Significant correlations between prostasome concentration and CD molecules were weak or lacking. Further, no or weak relationships were observed between the prostasomal CD markers and sperm morphology, seminal fructose, neutral alpha-glucosidase activity, zinc and tumour necrosis factor alpha concentrations. Flow cytometry is a practical way to study prostasomes in seminal fluid without prior separation. This is a new technique for evaluation of the role of prostasomes and their functions in male reproductive physiology.