Abstract: High-moisture absorbing and releasing fibers which have moisture-absorbing properties and moisture-releasing properties, are resistant to repetitive use, and have both flame resistance and antibacterial properties and a process for producing the same are provided.Acrylic fibers in which increase in nitrogen content by hydrazine cross-linking, an amount of salt type carboxyl groups and amido groups by modification of nitrile groups, tensile strength, LOI and sterilization rate are specifically adjusted. Such the fibers are obtained by hydrazine cross-linking treatment, hydrolysis treatment and conversion of the carboxyl groups to the salt type.High-moisture absorbing and releasing fibers having excellent processability in addition to flame resistance and antibacterial properties are obtained. Such the fibers can also be used repeatedly.