The authors report here a simple radioenzymatic determination of total L-homocysteine in plasma and urine. L-homocysteine-containing disulfides were reduced with dithioerythritol. L-homocysteine was then condensed by S-adenosyl L-homocysteine hydrolase to 14C-adenosine to form S-14C adenosyl L-homocysteine that was separated from 14C-adenosine by paper descendant chromatography. The concentration of the total plasma L-homocysteine of 45 normal subjects was 8.04 +/- 0.26 (mean +/- SEM) mumol/l and total L-homocysteine concentration in urine was 0.59 +/- 0.06 mumol/mmol of creatinine (25 subjects). This method is as sensitive than the other methods described in the literature but more rapid and less expensive.