Government of Ontario

CFS - Technical Information Sheets - Biology - Semen

Centre of Forensic Sciences

Technical Information Sheets


Semen Information – PDF, 26 kb (Rescinded Sept. 17, 2012)


Semen is a liquid from the male reproductive system that usually contains spermatozoa (sperm cells) along with various other substances. Mature males emit semen during ejaculation. The spermatozoon (singular of spermatozoa) is the male reproductive cell. Human semen usually contains high levels of a substance called acid phosphatase. Acid phosphatase has a chemical activity, which allows for its detection. Human semen also contains high levels of a substance called p30, which, with few exceptions, is specific to semen.

Examination For The Presence Of Semen

In general, the following steps are taken to determine whether or not semen is present on an item. Semen is first located by means of a visual examination and acid phosphatase testing. Microscopic identification of sperm cells or the detection of p30 confirms the presence of semen.

Tests For The Presence Of Semen

Visual examination - may involve the use of a stereomicroscope (a magnifying tool) and enhanced light sources.

Acid phosphatase testing - the presence of acid phosphatase on an item will give rise to a purple coloration upon addition of specific chemicals. The intensity of the color produced relates to the quantity of acid phosphatase present.

Testing for p30 - the liquid component from an extract of cellular material may be tested for the presence of p30. The detection of p30 is done using a commercially prepared reagent that specifically binds p30, allowing for its visual detection.

Microscopic examination - human sperm cells are identified in an extract of cellular material. Criteria for human sperm cell identification are as follows: size, shape and staining properties, following a treatment with chemical dyes.

  1. Semen is lost from body cavities in a variety of ways. The maximum reported time periods at which semen has persisted in living persons is as follows: 7 days in the vagina, 1 day in the mouth, 2 to 3 days in the anus/rectum. Generally, semen will not persist for these maximum time periods.
  2. The presence of spermatozoa on anal/rectal samples does not always indicate that anal/rectal intercourse has occurred.
  3. The non-detection of semen on internal samples is not in itself proof that a sexual act did not occur.
  4. The detection of strong acid phosphatase activity alone is not proof of the presence of human semen. Acid phosphatase is also found in other body fluids at lower levels. Post-mortem internal samples, heavy vaginal deposits, feces, urine, unusually high numbers of bacteria or yeast, plant extracts and certain chemicals may all show various levels of acid phosphatase activity.
  5. Acid phosphatase and p30 are water-soluble and can therefore be lost through contact with water e.g. laundering. Small numbers of spermatozoa can be retained on fabric after laundering.
  6. The presence of high levels of p30, with few exceptions, is specific to semen. The levels of p30 reported in the literature for other body fluids are comparatively very low, although high levels of p30 have been found in the serum of prostatic cancer patients and in rare instances in semen-free vaginal swabs.
  7. It is possible for a small number of spermatozoa to be deposited on items through innocuous means such as secondary (indirect) contact or laundering.


Ejaculation - An abrupt discharge of semen.

Pre-ejaculatory fluid - A small quantity of lubricating fluid, released prior to ejaculation that may or may not contain small numbers of spermatozoa.

Time Since Intercourse (TSI) - An estimate of the maximum time elapsed between intercourse and the collection of internal samples.

Vaginal/Oral/Rectal smear - A smear made from a vaginal/oral/rectal swab onto a microscope slide.

Vasectomy - A medical procedure performed in order to prevent the emission of spermatozoa during ejaculation. Commonly used for male sterilization.