The measurement of plasma S-adenosylhomocysteine is a more sensitive indicator of the risk for vascular disease than is plasma homocysteine. Because the level of S-adenosylhomocysteine is normally in the nanomolar range, it has been difficult to measure and necessitated the development of complex fluorometric and mass-spectrophotometric methods. We have now adapted an existing immunoassay used for the measurement of homocysteine to the measurement of S-adenosylhomocysteine in plasma. This assay is sensitive down to the level of less than 0.1 pmol, and there is no interference by S-adenosylmethionine. The assay is carried out in microplates, allows the measurement of 12 samples per plate and can easily be carried out in a 4-h period. The method is applicable to plasma samples having S-adenosylhomocysteine concentrations ranging from 10 to 150 nM without dilution. The mean value for 16 normal subjects by this method was 18.9+/-1.4 nM (S.E.M.), compared with 17.8+/-1.4 nM obtained by a previously described method using two high-performance liquid chromatography columns with fluorescence derivatization. Mean values for seven cirrhotic patients were 46.5+/-3.3 nM by this new method compared with 44.6+/-5.3 by the former method. The ease and speed of this method should allow the widespread measurement of this important metabolite in laboratories without access to sophisticated equipment.