Twenty-eight regularly menstruating female volunteers made careful collections of their sanitary pads and tampons on a daily basis throughout one menstrual period in such a way that the total fluid volume of the menstrual discharge could be accurately measured independently of the blood loss. The percentage contribution of blood (equivalent to mixed venous blood from the cubital fossa) to the total varied very greatly from woman to woman (1.6 to 81.7%) with a mean of 36.1 +/- 3.6% (+/- SEM). There was a highly significant correlation between total fluid loss and blood loss (r = 0.911, P less than .001). The proportion of blood remained approximately the same for different total volumes and on different days of the cycle. Women using no contraception or who had undergone tubal sterilization had similar ratios of blood to total fluid loss, whereas intrauterine device (IUD) users had a higher ratio (P less than .025) and oral contraceptive users a lower ratio that just reached statistical significance (P less than .05). It seems probable that the major component of the fluid loss that cannot be accounted for by blood is from endometrial tissue fluid rather than vaginal or cervical secretions.