An anti-Met-enkephalin serum has been used for radioimmunological and immunocyto-chemical demonstration of Met-enkephalin in planaria. 2. The serum showed a high degree of sensitivity and specificity. 3. Radioimmunoassay demonstrated in planaria extracts a considerable concentration of Met-enkephalin. 4. Met-enkephalin was localized by immunocytochemical methods in neurons and neuropil.