Zonae pellucidae of unfertilized hamster and human eggs remain penetrable by homologous spermatozoa when they are stored in highly concentrated salt solutions [Yanagimachi et al., 1979]. Neutralization of salt solutions with the appropriate pH buffers is recommended for long-term storage of the eggs (zonae), because zonae may gradually swell or even dissolve in acidic or alkaline solutions. At least for the hamster, 2 M ammonium sulfate solution containing 40 mM Hepes and 0.5% dextran (pH 7.0) seems to serve the purpose satisfactorily. Thorough rinsing of salt-stored zonae prior to insemination is important, but inclusion of protease inhibitors in the rinsing medium does not appear to be essential. A dialysis method is recommended when a large number of salt-stored eggs must be rinsed at one time. Salt-stored hamster zonae, like fresh (control) zonae, permit attachment and penetration of hamster spermatozoa, but not human spermatozoa. Salt-stored human zonae, on the other hand, allow penetration by not only human spermatozoa, but also hamster spermatozoa.