The sites of synthesis of the low molecular weight beta-trace protein, present in a seven times higher concentration in normal human CSF than in normal human serum, have been studied by means of a radioactive immunoprecipitation method. Adult squirrel monkey tissues were cultured in Eagle's minimum essential medium in the presence of 14C-labelled valine, threonine and leucine for 24 hours. Synthesis could be demonstrated in cultures of white CNS matter, whereas cultures of grey CNS matter, peripheral nerve, skeletal muscle, kidney and ovary did not show any signs of synthesis. Some cultures of spinal cord, basal ganglia, genital organs except ovary, and liver showed a probable synthesis of beta-trace protein. By means of autoradiography, the synthesis of beta-trace protein in white CNS matter could be confirmed.