An antibody directed towards ribosyl-trans-zeatin has been produced and characterized. The antiserum was produced in rabbits using ribosyl-zeatin-bovine serum albumin as an immunogen. A radioimmunoassay which employed this antiserum and a tritiated antigen was established. As little as 10 picomoles ribosyl-trans-zeatin could be detected. The specificity of the antiserum was measured in the radioimmunoassay by using nonradioactive nucleosides as competitive inhibitors. Changes in position N6 were more effective in decreasing antibody recognition than changes in position 2. Of particular interest was the interaction of the isomer ribosyl-cis-zeatin. This compound was significantly less active as an inhibitor than ribosyl-trans-zeatin, demonstrating that the antibody was sensitive to minor changes in the structure of the antigen.