The purpose of this study was to compare various seminal plasma parameters in fresh human ejaculates exhibiting different amounts of coagulum. Poorly coagulating samples demonstrated significantly lower osmolality and buffering capacity but had a higher pH than did samples with good coagulation. However, no correlation was obtained between the activity of superoxide dismutase and the amount of coagulum. Eight hours after ejaculation, sperm motility had decreased by 15 and 80% in samples with good and poor coagulation, respectively. It is suggested that subfertility may be associated with poor coagulation of ejaculates. Copyright © 1988, Wiley Blackwell. All rights reserved