Free and membrane-bound polysomes from rat liver were incubated with 8H-, and 14C-leucine, respectively, in a cell-free protein synthesizing system. The labeled supernatant fractions were then combined and the arginase fraction was prepared by an immunological method, i.e., by employing the specific antibody against purified rat liver arginase after removal of unspecific proteins. The results indicate that arginase [EC 3.5.3.1] in hepatic cells is preferentially synthesized by free polysomes.