(1) Mixed insemination of known numbers of spermatozoa from two rabbits produced, spermatozoon for spermatozoon, offspring in the ratio of 1:5. This difference in spermatozoan fertility was highly significant, and consistent over four experiments. It is thought to be a consequence of, and in turn a useful measure of, differences in the viability of spermatozoa in the original ejaculates. Such differences may be of common occurrence, and their existence would not necessarily be suspected from the records of natural matings. (2) The conception rate, or percentage of inseminations yielding a litter, was shown in one experiment to increase as the number of males contributing to the inseminate increased, even though the net total number of spermatozoa per inseminate was held constant. These observations are not fully conclusive, but are presented as a recorded experimental finding of the type expected by certain Russian workers. (3) Inseminates from one particular male alone gave no offspring, but offspring fathered by this male appeared after mixed insemination, as if the fertility of its spermatozoa had been assisted by the admixture with other semen.