The prevasectomy semen analyses of 1300 men who had fathered at least two children were studied. The ages of the men ranged from 23 to 64 years, with a mean age of 39.6 years; 84% of the men were Caucasian, 5% were black, and 5% were of other ethnic groups; 32% of the men were Roman Catholic, 26% Jewish, 22% Protestant, and 11% professed no religion; in 9% religion was not recorded. The number of children ranged from 2 (48%) and 3 (30.6%) to 10 (0.1%). The volume of the ejaculate ranged from 0.1 to 11.0 ml, with a mean of 3.2 ml plus or minus 1.4 SD. Sperm density ranged from 1.5 to 375 million/ml, with an arithmetic mean of 79 million/ml plus or minus 57 SD; however, the logarithmic or geometric mean was chosen as being more representative of the asymmetric distribution under study. The geometric mean was 65 million/ml, with standard deviation limits of 30 to 142 million/ml, figures that correspond to the 16th and 84th percentiles. The percentage of motile sperm ranged from 5 to 95%, with a mean of 65% plus or minus 22 SD. The grade of sperm motility, according to MacLeod and Gold's classification (Fertil Steril 2:187, 1951) of 0 to 4, ranged from 1 to 4, with a mean grade of 3. There were 25 men (2%) with a sperm count below 10 million/ml and 26 (2%) in whom the percentage of motile sperm was less than 20%. Polyzoospermy (count greater than 250 million/ml) was found in 15 men (1.2%); one or more spontaneous abortions had occurred in six of the spouses of these men (40%), whereas the over-all spontaneous abortion rate was 7% and the spontaneous abortion rate for wives of men with sperm counts below 10 million/ml was 4%. A positive relationship was found between sperm density and percentage of motile sperm; the relationship between motility and the logarithm of sperm density was represented by a straight line (a 10-fold increase in sperm count was accompanied by an increase of 20.4% in sperm motility); the slope of the line was highly significant, although the correlation coefficient was weak (0.27). A similarly positive relationship was found between sperm density and grade of motility; however, the correlation coefficient was also weak (0.22). The relationship between the grade of motility and the proportion of motile sperm also positive, was reasonably large (0.70), although its significance is questionable because of the crudeness of the scale utilized to assess the grade of sperm motility. A comparison of the findings with those of similar studies was made.