Abstract: High moisture-absorbing and releasing fibers exhibiting excellent moisture-absorbing properties and moisture-releasing properties, capable of withstanding repeated use, having both flame resistance and antibacterial properties, and further having excellent whiteness and workability, as well as processes for their production, are provided. These fibers are made from acrylic fibers and have been particularly adjusted to have an increase in nitrogen content by hydrazine crosslinking, amounts of salt type carboxyl groups and amido groups, tensile strength, limited oxygen index (LOI), sterilization rate, amount of heat evolved by moisture absorption, and whiteness. The production of these fibers are achieved by hydrazine crosslinking treatment, acid treatment, alkali treatment, and conversion of carboxyl groups into those of the salt type. The above fibers can be used for various purposes and can find an enlarged range of applications.