Several antiviral substances have been detected in human serum but few have been shown to possess broad antiviral activity. These broadly active antiviral molecules could be of significance as innate defense mechanisms. We have previously identified and characterized a broadly antiviral glycoprotein, UTI3, which accounts for 50 antiviral units/ml of human and mammalian sera. In addition there are reports of antiviral activity of human serum apolipoprotein A-1 (apo A-1), an important constituent of high density lipoprotein (HDL), against human immunodeficiency virus (HIV) and herpesvirus. Therefore we investigated (1) whether HDL is broadly antiviral, (2) how much of the broad antiviral activity of serum is due to HDL, and (3) the mechanism(s) of HDL's antiviral action. In this paper we report that (1) HDL does have broad antiviral activity, (2) HDL accounts for a modest but significant portion of the antiviral activity of serum, and (3) HDL acts by preventing virus penetration. Overall, HDL may be one of the broadly antiviral defences in the bloodstream.