The principal zinc binding component in human prostatic fluid and seminal plasma has been found to bind zinc reversible and to be of low molecular weight. Treatment of this zinc containing complex (LMW-Zn) with trypsin and pronase or acid hydrolysis failed to alter its zinc binding properties or gel chromatographic characteristics. Removal of citrate with a combination of citrate lyase, malate dehydrogenase and lactate dehydrogenase did, however, completely abolish its ability to bind zinc. Spectrographical scanning of LMW-Zn preparations obtained by gel chromatography of prostatic fluid or seminal plasma on a Sephadex G-25 column gave identical absorption curves with those obtained by scanning of a zinc-citrate solution. Application of the same analysis to LMW-Zn of human breast milk yielded similar results. It is concluded that citrate is the major low molecular weight zinc ligand in prostatic fluid, seminal plasma and breast milk.