A study in vitro of interactions between gametes of the golden hamster showed that the spermatozoon associates with the zona pellucida, or outer coat of the ovum, by two successive steps termed attachment and binding. Attachment, apparently the first step in fertilization, is not species-specific; it is insensitive to temperature (2 degrees ) and is reversible. Binding, on the other hand, is species-specific, temperature-sensitive, and irreversible. Experiments with isolated zonae pellucidae indicated that the vitellus (cellular portion of the ovum) influences the interaction between the sperm and the zona pellucida by increasing the time required for the sperm to bind. This effect is exerted on the sperm while they are attached to the zona pellucida.