The highlights in the development of frozen semen have been covered by other reviews and reports. Interested persons will find the articles of Polge and Parkes[2] and Smith[4] especially good on the early history and theoretical aspects of freezing sperm. Later progress on the freezing procedure has been reviewed and covered in a number of detailed reports.[5],[6],[7] Many items not covered in those articles have been assembled here. SELECTION OF SEMEN FOR FREEZING One of the first considerations in freezing semen is that of deciding which semen samples are to be frozen. Since preservation of the semen--the maintenance of the potential motility and especially the fertility of the sperm--is the primary aim, some attention should be directed to the kind of semen sample that will withstand freezing. Do the initial characteristics of the sample indicate whether the sperm will withstand freezing? Does maturity of the sperm affect their freezability? =Predicting freezability.= Estimates of semen quality in the past have been based in part on the numbers of sperm present in a fresh sample and on the percentage and rate of motility shown by the sperm. These characteristics were used to determine the relationship between the original concentration of sperm (in the fresh, undiluted sample), the percentage and rate of sperm motility in the diluted samples just prior to freezing, and the percentage and rate of sperm motility following freezing and thawing. From data collected before and after freezing and thawing 54 ejaculates, it was found that there was not a significant correlation between the number of sperm present in the original sample and the percent of motile sperm present after freezing and thawing (r = 0.03). A highly significant correlation (r = 0.45) was found, however, between the percentages before freezing and after thawing. While this correlation coefficient was highly significant, its magnitude indicates that only about one fifth of the variation in percentage of motile sperm observed after freezing was accounted for by the motility of the sperm prior to freezing.