Proteins of Atlantic salmon (Salmo salar L.) spermatozoa were lost into the seminal fluid during storage at −80 °C. Addition of glycerol or dimethylsulfoxide reduced but did not completely prevent loss of sperm proteins. Of the two, glycerol more effectively fostered retention of sperm proteins, especially those of higher molecular weight. Some of the proteins lost during storage were surface labelled via lactoperoxidase-mediated iodination. More protein species were labelled in frozen than in fresh spermatozoa. The massive loss and extensive labelling of sperm proteins with 125I are taken together as an indication of damage to salmon sperm membranes during freezing, storage, and thawing.