In mammalian spermatozoa, the element zinc is mainly located in the outer dense fibers (ODFs), which are important for the generation of motility. For normal formation of the ODFs during spermatogenesis, zinc is bound to cysteine, which is present in ODF proteins to protect them from oxidation. During epididymal transit of spermatozoa, this element is eliminated from the flagella. By this, disulfide bridges are formed in ODF proteins, which stiffen this substructure, leading to better use of the energy. To investigate the importance of zinc for sperm motility, we measured the zinc content of human sperm by means of atomic absorption spectrometry in relation to motility. To get an insight into how zinc is removed from the ODFs during epididymal transit, bovine epididymal fluid from caput, corpus, and cauda was labelled with [sup 65]Zn, and radioactivity was measured after gel filtration. Proteins of epididymal fluid were also analyzed by gel electrophoresis with subsequent silver stain. Our results showed that the mean flagellar zinc concentration (37.3 ng/10 6 spermatozoa) is negatively correlated with motility (r = -.484; p <.001). Chromatographic experiments revealed a zinc-binding protein with an apparent molecular mass of 150 to 160 kDa, which is co-located with the peak of radioactivity. Under reducing conditions, two distinct protein bands with apparent molecular masses of 80 and 60 kDa were observed. [ABSTRACT FROM AUTHOR]