SR 121 is one of the major routes of a three-state highway 121 that totals approximately 475 miles (764 km) from Lebanon Station, Florida, to
Rock Hill, South Carolina. Beginning in Lebanon Station at a
channelized intersection with
US 19/
US 98 and
County Road 336 (CR 336) in
Levy County, SR 121 travels through
Goethe State Forest where it turns east and intersects
CR 337. The road makes a sharp left curve to the northeast where it travels through desolate farmland; the only resemblance to a major intersection is
CR 326. Within the city of
Williston, SR 121 has a
concurrency with
CR 316, until the intersection with
US 41. It then joins US 41 as CR 316 continues eastward, which then joins the east end of
US 27 Alternate (US 27 Alt.), and then the
US 27/US 41 concurrency before breaking away to the northeast again, taking the DeSoto Trail away from US 41. North of
CR 335, SR 121 hugs the west side of the Levy–
Marion county line, but never crosses it, choosing to enter
Alachua County instead. Immediately after the
interchange with
Interstate 75 (I-75) at exit 382, SR 121 makes a sharp left onto Northwest 34th Street as
SR 331 becomes the designation for Williston Road. As it crosses
SR 24 it borders the western edge of the
University of Florida, passing along the
34th Street Wall just before crossing
SR 26A and then
SR 26. North of
US 441, the DeSoto Trail breaks away. Within the town of
LaCrosse, SR 121 shares a short concurrency with SR 235, then has a level crossing with the CSX
Brooker Subdivision, a former
Seaboard Coast Line railroad line that leads to the
Crystal River 3 Nuclear Power Plant. After the intersection with
CR 236 in Santa Fe, SR 121 passes by entrances to the Santa Fe River Preserve, and then is carried along a bridge over the
Santa Fe River in
Worthington Springs, where it intersects
SR 18. North of CR 18, the road is carried along a bridge over the former
Jacksonville and Southwestern Railroad. Briefly traveling parallel to
SR 100 in Lake Butler, it intersects SR 100 as it continues northeast. The road also runs parallel to another rail-trail along the former Jacksonville and Southwestern right-of-way in
Raiford, then serves as the western terminus of
SR 16 before leaving that community. After passing by the Northeast Florida State Hospital, the road intersects
I-10 (Exit 335) and
US 90 in
Macclenny close to
SR 228 before crossing the
Georgia state line on its way to
Rock Hill, South Carolina. ==History==