A comparison was made between successful (167) and unsuccessful (1322) insemination cycles in order to evaluate the role in conception of different semen characteristics. The most important semen variable was found to be post-thaw motility: the success rate per cycle increased from 7% when post-thaw motility was less than or equal to 40% to 17% when it was greater than or equal to 65%. Multiple inseminations in a cycle increased the success rate primarily when semen quality was poor. The results of this study show how and to what degree efficacy in A.I.D. can be improved by a judicious choice of the semen to be utilized and the number of inseminations to be practiced.