each particular animal species will be situated near
the centre of this region. The optimal dilution factors
thus found were: Boar semen + x25; Bull semen + x100;
Cock semen + x200; Human semen + x50.

All curves show exactly the same functional shape, as
is demonstrated most clearly after normalising them by
a scale shift where they coincide. Fig.2 shows the
normalised curve for all data of the four species,
presented in the form of log (transmission)=f(concentr-
ation).

Obviously the only difference between species is the
aequivalent absorbing area per smermatozoon, which is
governed by the concentration of the UV-absorbing cell
constituents per unit area, i.e. mainly by the amount
and spatial distribution of DNA. This implies that the
calibration curve obtained for any animal species can
be applied to any other species as soon as the proport-
ionality factor between the two specieg is known.
Readings need only to be multiplied by the calibration
constant, or the scale is shifted relative to the curve.

The relative calibration factors in our investigation
were: Man 1.00 (per definition); Boar 0.87; Bull 1.60;
Cock 0.15.

Semen dilution is performed accurately, using micro-
pipettes and measuring flasks. Part of the mixed dil-
ution is centrifuged. Centrifugate and semen dilution
are brought into quartz spectrophotometer cuvettes,
the centrifugate being used as the 100% transmission
standard; = 260 nm + 5 nm. All measurements are per-
formed in duplo. Using a single-beam photometer with
an interference filter for selecting the required wave-
-length the overall accuracy was 10% in the transmiss-
ion values, corresponding to 11-12% in sperm densities.
This is of the same order as obtained with duvplicated
haemocytometer countings of 300 sperms per chapber
(1,7), but the time required was only 1/3. The nain
part of the error (46%) was due to the dilution proce-
dure. Cuvette error was 2%, within chance expactation.
There was no difference between technicians.

For determination of sperm density of bull semen
the UV-absorptiometric method has no advantage over
direct absorptiometry in the visible regior, neither
with respect to time consumption nor to accuracy.

The advocated field of application is restricted to
semen of species with highly turbid seminal plasma.

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