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Summary. Glycerolated semen specimens of several donors were examined for DNA content after refrigeration at +6\s=deg\ C for 7 days, and after storage in liquid nitrogen for periods ranging from 2 to 75 weeks. Fresh specimens were also studied. DNA determinations were made in each case by both Feulgen microspectrophotometry (in arbitrary units) and by the Webb-Levy chemical determination. The quantity of DNA per cell in arbitrary units and in mg \m=x\10\m=-\9remained constant after refrigeration, and after freezing-preservation for all time periods studied. Discrepancies between the two kinds of determination, and the concept of embryonic mortality due to aged spermatozoa are discussed.

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A serious difficulty in the use of frozen-preserved human semen for donor insemination is our inability to predict, without rather extensive clinical trials, which specimens will be successful in producing conception and which will be generally infertile. Predictions cannot be made from the pre-freeze motility of the specimen (Behrman & Sawada, 1966) nor, in our experience, from the fertility of a donor's freshly ejaculated specimen. A technique for determination of cervical mucus penetration by spermatozoa using capillary tubes (Kremer, 1965) has been thoroughly investigated and found to have several advantages (Kremer, 1968). This capillary tube test has been shown to be a reasonably reliable predictor of the fertility of fresh human semen (Fj\l=a"\llbrant,1968). Specifically, a penetration by the leading spermatozoa in the capillary tube of more than 5 mm was found to be a necessary condition for fertility. Thus, it appears that this test may be a good screening device in the selection of frozen-preserved specimens for clinical use. To make the test still more reliable and useful, some modifications were