For a Type III or Type IV building, can non-treated studs and sheathing in a fire-tested assembly for exterior walls be replaced with fire retardant-treated studs and sheathing?

Yes, wall assemblies that were tested with untreated wood studs may instead use fire retardant-treated wood studs. The UL Fire Design Information Section, Part VI: Walls and Partitions, Sub-section 1: Wood Stud Walls, states the following:

“Wood stud walls may contain fire retardant-treated studs as well as untreated wood studs. The use of fire retardant-treated plywood (wood structural panels) may be used in designs that contain use of untreated plywood when all other specified attributes are equivalent to the wood structural panel used in the design.”