Oxidizing agents, such as potassium ferricyanide, sodium tetrathionate, iodine, hydrogen peroxide, sodium periodate, chromic acid and potassium dichromate, cause hardening of the chorion. Chromic acid is less effective than any of the other agents. In the presence of reducing agents, such as sodium sulfide, potassium cyanide, sodium thiosulfate, sodium sulfate, sodium thioglycolate, ammonium sulfate, and potassium ferrocyanide, the chorions fail to harden. Treatment of unfertilized eggs with mercuric chloride or p-chloromercuribenzoate induces the inhibition of chorion hardening when they are subsequently inseminated in Ringer''s solution. The same result is obtained also in the cases where both agents are applied after fertilization. Either inhibitory effect is removed if the treatment is followed by that with potassium cyanide. Iodoacetic acid and chloroacetophenone likewise inhibit hardening when eggs are treated after insemination. Neither urea nor lithium bromide has any effect on the chorion hardening. Aldehydes, such as formaldehyde, acetaldehyde, acrolein and benzaldehyde, are capable of hardening the chorion. Soft chorion is impregnated with a protein containing SH groups, together with polysaccharide which has [alpha]-glycol groups. It is concluded that the oxidation process is involved in the mechanism of chorion hardening. It is suggested that hardening is due to combination of SH groups with the aldehydes produced by oxidation of [alpha]-glycol groups.