The role of serum albumin in in vitro fertilization of mouse ova was studied relating to the fertilization inhibitory activity of Zn²⁺. The time required for mouse sperm capacitation was dependent on the concentration of serum albumin which was added to the medium. The majority of Zn²⁺ added to the medium was shown to be bound to serum albumin (95% out of 200 μM) by means of equilibrium dialysis. Furthermore, it was suggested that the serum albumin was necessary for the initiation of the sperm capacitation process by the periodic observation of sperm penetration through zona pellucida.