The metabolites of L-[methyl-11C]methionine in the plasma of 8 patients with tumors were measured for 60 min after injection. In the plasma, after a rapid clearance, the total radioactivity remained constant, and protein-bound radioactivity increased rapidly. Non protein metabolites detected by HPLC as at least two components besides methionine, increased with time. Significant individual variations for the metabolism were observed. At 60 min after injection, 36.5% (range: 16%-72%) and 45.3% (range: 13%-74%) of the 11C was measured as methionine and labeled proteins, respectively.