The diluted semen was equilibrated for approximately
24 hours at 5°C. The straws were filled, sealed and finally
frozen vertically in liquid nitrogen vapour (4) before storage
and distribution in liquid nitrogen. The second trial differed
from the first only in that 5 x 10° spermatozoa per insemination
were used instead of 30 x 10® and the employment, with one
exception, of different inseminators.

RESULTS AND DISCUSSIONS.

The results of the two trials are summarised in the
following table :-

Eirst iTrial Second Trial

No. Spermatozoa Firsts C/R* Firsts G/R%

30 x 106 557 78.6

20 x 106 527 78.6 550 72:3
10 x 106 565 79.5 533 74.5
5 x 106 - - 532 76.5

* 28 day non-return percentage

There is no evidence from these trials that a reduction
in spermatozoa numbers per insemination from 30 to 5 millions
had any effect on the 28 day non-return rate percentage when
using semen frozen in 0.5 ml. straws. It will be noted,
particularly in the second trial, that the overall conception
rate was a little lower than that which is considered desirable.

This work was based on total spermatozoa per ml. because
the authors prefer an objective assessment of numbers to a
subjective estimate of motile spermatozoa, particularly as the
true characteristics of spermatozoa cannot be gauged
microscopically. These studies are based on the assumption that
the semen collected for freezing and the processing techniques
will be, at least, of a high standard.

Since this is an interim report too much should not be
read into these results but the studies will continue till the
smallest number of spermatozoa compatible with normal fertility
is reached. Then, using the experience and confidence gained,
larger scale field fertility trials involving much greater

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