Human prostasomes isolated from seminal plasma were subjected to phenol extraction and then to absorbance (A) measurements at 260 nm (A260) and 280 nm (A280). The A260/A280 ratio was about 2 for prostasome extract and lower for seminal plasma extract, indicative of the presence of nucleic acid. The ratio of nucleic acid to protein in prostasomes was about 1:100, and the ratio in seminal plasma was 1:1,000. Hence nucleic acid is enriched in prostasomes (compared to seminal plasma of 10). Treatment of prostasome samples with 2% sodium dodecyl sulfate resulted in an efficient dissociation of nucleic acid from prostasomes as demonstrated by electrophoresis. The association of nucleic acids of various sizes (range; 200 to 20,000 base pairs) to prostasome membranes was most probably genuine and not the result of contamination from spermatozoa, erythrocytes, leukocytes, or bacteria. The results of experiments employing nucleic acid-degrading enzymes favored the concept that double-stranded DNA but not RNA is present at the prostasome membrane surface.