A comparative study of the sialic acid content of semen, serum and spermatozoa from several mammalian species was made. The sialic acid content of semen showed a very wide range of concentration in the four species examined, while the sialic acid concentrations in serum were much more uniform. The semen from man and bull contained higher concentrations of sialic acid than the serum of these respective species. The reverse was found in rabbit and dog.
Only a small fraction of the total sialic acid in semen was found unbound and spermatozoa were found to contain very little sialic acid. The sialic acid of semen and serum was readily accessible to the action of Receptor Destroying Enzyme (rde) except that of rabbit semen. Using paper chromatography n-acetylneuraminic acid was identified in all the samples. n-Glycolylneuraminic acid was also observed in bovine serum and a trace was found in rabbit serum.