Plasma samples of 189 healthy subjects were investigated for antigen levels of the recently reported factor VII- and single-chain plasminogen activator-activating protease (FSAP) and the corresponding pro-urokinase activating potencies. While the age of donors had no significant effect on the investigated parameters, female plasmas revealed a trend to higher antigen contents and activity levels. Surprisingly, as much as 9% of all samples contained significantly reduced single-chain urinary plasminogen activator activating potential, whereas antigen concentrations were normal. Additionally, 1% of the plasmas was found to decrease in both FSAP antigen and activity contents. FSAP of three subjects displaying reduced activities throughout a follow-up period of 6 months were purified from plasmas and were characterized. As compared with pool plasma derived FSAP, investigation of the individual preparations confirmed their reduced potency to activate pro-urokinase. However, factor VII activation was not affected. It is speculated that the FSAP binding site for single-chain plasminogen activators is affected, potentially by as yet unknown polymorphism(s) or mutation(s).