Preparation of the antibody for radioimmunoassay of epinephrine was examined and an antibody with proper titre was obtained. A procedure for radioimmunoassay of epinephrine was developed by use of a membrane filter to separate bound and free hapten. The dose-response curves of analogs of epinephrine obtained by this method showed that the specificity of the antibody to epinephrine was sufficiently high : The 50% inhibition value of epinephrine was 1 pmol, those of metanephrine and synephrine were 80 pmol and those of norepinephrine and dopamine were 6×10² pmol. The result indicates that the antibody recognizes not only the side chain but also the catechol moiety of the hapten. The sensitivity of this radioimmunoassay was high enough to determine 0.1 pmol of epinephrine.