Human serum albumin was able to hydrolyze the organophosphorus compounds paraoxon, chlorpyrifos-oxon, and diazoxon at toxicologically relevant concentrations. Human serum displayed two paraoxon hydrolyzing activities: the so-called paraoxonase, which is associated with the lipoprotein fraction and is calcium dependent and EDTA sensitive, and the activity associated with albumin, which is EDTA resistant and sensitive to fatty acids. Human serum albumin hydrolyzed these compounds with the same relative efficacy as lipoproteins (chlorpyrifos-oxon > diazoxon > paraoxon). The capability of detoxication of activity associated with human serum albumin was similar or even higher than paraoxonase associated with lipoproteins in the case of paraoxon at concentrations as low as those noted in an acute in vivo intoxication. However, paraoxonase activity associated with lipoprotein was more effective than paraoxonase activity associated with albumin at toxicologically relevant chlorpyrifos-oxon concentrations. These results explain why mice deficient in paraoxonase associated with lipoprotein are not more sensitive to paraoxon than wild animals.