The major glycoprotein on the plasma membrane of testicular spermatozoa labelled with the galactose oxidase/NaB3H4 technique has mol.wt. 110 000. As spermatozoa pass through the epididymis, labelling of this glycoprotein disappears and is replaced by labelling of a 32 000-mol.wt. protein. The latter protein is a major component of epididymal secretions. The evidence suggests that it is inserted into or absorbed on to the plasma membrane, and since its appearance on spermatozoa correlates with the acquisition of fertilizing capacity it should serve as a good marker for assessing maturation in vitro .